I have been making and retrofitting my own hand grips. I mak a male mould with shaped polystyrine covered in melted and smoothed wax (smooth with the edge of a hot knife), wrap it in about 6 layers of 4oz cloth with pigmented epoxy (polyester resin would be OK too). When hardened dig out foam and wax, sand off till smmoth and all traces of wax removed. Trace out bottom of handle on deck, cut hole in deck glass with Stanly knife cut out enough internal foam to allow the hand grip to sit flush and fix in with thickened epoxy (polyester resin cannot be used for this but use Araldite epoxy if you dont want to buy boardmaking epoxy).
You can also buy hand grips from Shapers.com.au these are good, but heavier and deeper (hold more water) than my home made versions. These can be put in with epoxy and are easy to fit (trace around base of handle, cut deck with Stanly knife, cut out foam and fix in with thickened epoxy, sand down to deck level)
They also sell gortex breather vents and it might be worth adding one of these as well (use the same method as above, but make sure you dont get epoxy on base of breather).
Hope this helps
Bill