I have the iShoot grip and L-bracket, and find it very satisfactory. Major improvement in handling. Fits very nicely. Grip is just the right size. Price on eBay was a fraction of what a UK site charges.
It had a couple short edges that they forgot to round off, but I did that myself with a Dremel-type handpiece, then re-blackened with aluminum gun blackening I had around.
The grip is aluminum and yes on first contact it feels cold, but if one holds it for a while it acclimatizes.
I was researching on coating it with truck-bedliner, but because it is aluminum, it would need to be coated with 2k primer first to ensure a bond, so I will leave it for now.
If it were patterned it would feel even better.
Another option, as someone mentioned above would be using modelling clay for a real custom grip. The best stuff for this is the Staedtler FIMO Pro modelling clay, which can be baked in an oven for further use in molding techniques. I plan on re-casting just the grip part with some Urethane Casting Resin.