I've used silicone sealer squeezed between the saddle of the riser & my hand to build it up & form fit it to my hand as a temporary thing.
Wrap the saddle with saran wrap first, slap some sealer on it, then put saran wrap on your hand, spray it with a releasing agent, (I used "pam" cooking spray) then hold the bow normally to form it to yer hand.
If you have to fill in a lot of space, better to do it a few stages letting the sealer dry inbetween.
Slowly remove the top layer of saran wrap (next to yer hand) & let dry. The sealer won't cure without air.
Once dry, I temporarily wrap it again with saran wrap to hold in place while I shoot the bow some to see how it feels etc.
If I decide it's worth the effort to have it permanent, I will carve some wood to match my silicone form & glue it on. If not, but still want the high grip, I'll glue on lifts of leather with contact cement after first applying paste wax to the bow wood so it is easily removed at a later date.
The things ya think of during a no-hunting time!!!
laffs