This is genius!
I've been looking a precisely the same thing for my Surface pen! I resorted to using the following vendor to buy 'free sample' 'sleeve grips' that I would modify for the pen buttons. The site does textured grips in 8mm and 9.53mm inner diameter that are textured for easy grip. I think that they are 5mm thick.I would have to make cut outs for the two buttons which would be fiddly.
http://www.componentforce.co.uk/category/209/round-sleeves---textured
I prefer your option however! It looks much more professional... as I live in the UK, postage from the US would be expensive for such a small item. Would you be happy sending me the STL file so that I can get it printed in London? If the cost is cheap enough I will offer to print out 10-20 and sell them (at the price is costs me to print and post) on for other UK people to test. That way we become beta testers for you as a 'thank you' for you initiative!
Let me know what you think by messaging me.