happy hours

I finally bit the bullet and bought some trim tabs for my 20' Chris Craft Bowrider. Because the molded in swim platform/ladder I went with Volvo QL tabs. (lower profile). Since the transom is slightly curved the tab does not sit totally flat. Paperwork just says to make shim to provide a flat mounting surface, no mention how to do so. If the center touches the gap on either end is ~1/4" maybe 3/8". Anyone have any experience making or doing this? I was 1/2 thinking about making a mold and pouring resin and thinking agent (like cavasil) and making a shim that way.

Anyone have any advice?