Castings not a bad option. One possible advantage of resin though is it can be coloured so you don't have to 're black the sights if they get rubbed or wear. Alloy has dome advantages too. Done lots of grips in resin, black or mottled brown, some odd ones too in creamy ivory colour with a vein in it. Made a few bright coloured ones too look ok till you put them on a pistol then they look a bit out of place. Everything's worth a try I suppose.

Small parts with large moulds can sometimes take the heat away, cooling the molten metal giving a wavy pattern, in the little casting I've done I found a quicker pouring technique was better but obviously you've got to beware of the risks if it splatters or you get a blowback from trapping air. Not so problematic with resin but then some of them set almost as soon as they are mixed, even without an acellerator/catalyst.