Just some updates on this topic... I customized a grip today, and had lots of fun. Took forever long to do it, cause I can't do anything the easy way. But anyway:
1) Put on a heat shrink sleeve using an electric burner on my stove. It came out crooked on top, so I trimmed it, protecting the paint job by sticking a scrap piece of tubing under the top as I cut it... but that stretched out the top. So I took it back to the burner, trying to get the top to shrink. But I forgot to rotate... and promptly burned a hole in the side of the tube. Oops...
2) Used the *other* heat shrink sleeve (which I had been saving for my other racquet) and did a better job. By the way, you need to put it more than 3/8 in. above where you want the top... more like 3/4 in. It shrinks a lot. I trimmed the bottom to where there is only a very small ring around the butt. [This brought it up one size]
3) Put vinyl tape over the top of the sleeve, then a few layers above it, since it hadn't gone as high as I wanted.
4) I had cut strips of 1/32" bass wood (harder and heavier than balsa) to the length I wanted (about 6 5/8 from butt cap to where I wanted the handle to stop) - 1/2 in. for the short sides and the diagonal bevels, and three times 1/4" for the long sides. Using medium CA glue (moving quickly), I started by gluing on the 1/2 in strips to the short sides. Then I had to trim the strips for the bevels down to about 7/16" with a box knife and scissors, plus sand them here and there, and glued them next. Then I put two 1/4" strips on each long side up against the bevels. This left an irregular gap on each long side, due to the size of those sides, imperfections in my technique, the shape of the top of the handle, etc. So I cut and sanded some tiny strips (about 3/8") to fit as best I could.
5) Then I sanded all the rough edges, trying to make it a smooth octagon.
6) Almost applied a coat of clear enamel, but thought better of it when I saw that it would need to dry for a few hours. Decided the wood was hard enough (it is bass, not balsa).
7) Wrapped the whole thing in packing tape (clear, thin tape) nice and tight, about 2 or 3 layers in several rows.
Built up the butt capp with vinyl tape, about five layers in two rows.
9) Reapplied the original grip and one overgrip.
Now it measures a little over 5 7/8 around. I still can't fit my whole index finger between my finger and my hand, but I like this size. I am wondering if grip sizes don't exactly correspond to 8ths of an inch, because the heat shrink, thin wood, and overgrip should have only added about 3 sizes, but they aded more than 4 (started out with a size 3).
And all the plastic, wood, and glue weighed so much that my Hyper Hammer 6.3 OS is now 1 point head-*light* (started out about 5 pts head heavy). We'll see how it plays... it feels good in my hands anyway.
Well, that got kind of off-topic for this forum. So to summarize what is relevant for heat shrink tubing:
1) It really will burn (and evaporate apparently). So be careful. Keep it moving.
2) But stoves do work, if you're careful.
3) Leave about 3/4 above where you want the top of it to be (for the stuff TW is currently selling anyway). It will shrink a lot. You should hold on to it when you start heating the bottom, but soon you will have to let go, and it will pull down.