I don't know if this idea for modifying your rod handle is something you would want to consider doing. It's not for the faint of heart.
Basically you remove the existing handle, re-wrap it with nylon line. Making a very long grip. ( you hold the rod such that the butt end is near the elbow, mark the place where the thumb reaches, and wrap it with nylon line to that point ). The line is double or triple wrapped, with the goal to wrap it with enough material to counter balance the rod when finished. The line is glued and then over wrapped with tennis racket wrap. If the handle needs to be larger in diameter, more than counter balanced, perhaps wrapping with a lighter material before over wrapping with tennis racket wrap, or some other type of wrap, would be better. Anyway, maybe the basic idea can be adapted to better suit an individual need. Clearly you would want to use something that doesn't absorb water, making it heavier and hard to dry.
This is not my idea. I know of maybe three Japanese web sites that attempt to present a complete online course in tenkara fishing. This idea is from one of those web sites.
There are 3 pages showing how he modifies the grip section of his rod, a 4.5m rod. The translation is from Japanese and leaves a bit , or maybe a lot to interpretation. For example, the text appears to say he wraps the rod to a length of 60cm, till he gets the counter balance about right. Then wraps it to 65cm. But that makes no sense to me. Somewhere around 40 -45 cm would seem more correct. 65 cm would be almost the full length of the first section of most rods. If you look at the web page where he shows how he cast, he holds the rod at a high point. Stopping the forward cast when the butt end of the rod hits the lower part of his forearm. A little different from the way casting is usually taught.
Testing the center of gravity point during the wrapping phase, the rod is setting on a balance point, and I assume the rod is fully extended :
He doesn't seem to put any effort into making the handle grip be contoured in anyway. Just long for counter balance. ( I have noticed that some other brands of tenkara rods also don't have contoured grips, they are just long) I'm not completely sure the rod he is modifying started life as a tenkara rod. It appears he says he chose this rod because it has a lillian, pike, and 1.1mm tip and it is therefore suited for tenkara. Or maybe that is just a weird translation.
Link to first page of grip modification, just follow the links at the bottom to the next two pages showing the process:
Source web page:http://kamihusa.biz/329.html
Translated web page:http://translate.google.com/translate?sl=auto&tl=en&js=n&prev=_t&hl=en&ie=UTF-8&layout=2&eotf=1&u=http%3A%2F%2Fkamihusa.biz%2F329.html
A couple of notes:
1) The translation says that he wraps the rod with "yarn octopus", タコ糸 = Tako ito. Anyone who frequents a sushi restaurant knows that Tako is for octopus roll. If I google タコ糸, I get lots of pictures of various kinds of twine or line. One of which was this one , which says Tako Line - nylon line, natural color. Thus I suspect that 'yarn octopus' is just what is says or any nylon line.http://www.gosen-f.jp/products/list.php?id=1332324143-560725&ca=7
2) he cautions when gluing the nylon line to, not breath the fumes and just as important , the glue generates heat when applied, so don't apply the glue to quickly.
Link to the 4th page where he shows how he cast with this modified handle rod. There are several still pictures and a short 19 second video. I'd link directly to the video but he does not have the video publicly on youtube. It is only linked to from this instruction page and I believe I should not bypass the way he wanted the video to be available.http://kamihusa.biz/416.html
Maybe a useful or idea, or maybe not. A big disadvantage would be making the rod heavier.
Now for something a little different and mostly off topic:
This is from a web page for salt water rods. But if you have the patience for wrapping handles with line, I don't, you can perhaps get inspired to make some unique wraps with colored line into interesting patterns. Unfortunately they don't show how they finish the ends of this type of handle wrapping. I can only think it must be glued and covered with parallel wraps of line. Click the drop down next to "How to Wrap". Maybe useful for a unique Tamo handle, with more earth tone colors.http://www.geocities.jp/fish2flower/
And this - A complete online course in Tenkara fishing. Its all in Japanese and its a pretty tough going trying to understand a lot of it. Especially the pages that are all text. I beat my head against it ever now and again. ( for fun
) That being said there are some really good diagrams that illustrate casting, how to determine what you're doing wrong and how to do it correctly. And other topics too with good illustrations. I think there are 30 chapters in total. Maybe worth a look. If you can stand trying to understand difficult things.
Source page, which for some reason only displays a mass of ascii characters on my computer, I suppose I'm missing some kind of plug-in for certain type of Japanese character encoding.http://www.ikufuan.com/tenkara/turiindx.htm
Translated page, which does display the odd English translation just fine. And for some odd reason also allows me to copy and past short bits of the Japanese characters into another translation window when I'm trying to make more sense of what its trying to tell me . http://translate.google.com/translate?sl=auto&tl=en&js=n&prev=_t&hl=en&ie=UTF-8&layout=2&eotf=1&u=http%3A%2F%2Fwww.ikufuan.com%2Ftenkara%2Fturiindx.htm