First, don't use regular styrafoam that dents, sheds cut particles, and leaves a ragged edge. Use "block foam" that won't do any of these things cut to fit in case. Make a cardboard cutout of your revolver- not the easiest thing. I traced slightly larger that the cutout shape in a "Stagecouch Box". Spray foam with "Elmer's Craft Bond Spray Adhesive". Immediatly lay cloth on foam and press revolver into cloth covered cutout area pulling and stretching to remove wrinkles (hard to get them all out). That's what I did-maybe others have better suggestions.
Hancock Fabrics and JoAnn's Fabrics are helpful, as well.
Make a 'well' out of corrugated cardboard that the piece can nestle down in - 'then' apply your fabric-stretching skills, and be sure to ask the 'craft ladies' how they'd approach the problem - they can be seriously helpful.
Talk to a trophy shop, as well.
'My' question is where to get that 'egg crate' foam found in those Doskocil gun cases - none of those places have that stuff...