Resin doesn't have much strength to it without cloth, I would suggest you glass it properly with a cloth resin mix (kitty hair) or stretch it with cloth and lay it up. Some people bond glass to ABS while laying it up by drilling holes in the abs so the resin can drip through and form a mechanical bond. Others prefer to bond a separate fab'd glass piece with 3M Duramix 40/40 or plastic epoxy. There are several ways to skin the cat with varying results. You could also tape it off, lay up the amp base, pull it and then use screws etc to mount it to the ABS piece. Remember if an amp is hanging off it while going over the bumps, some decent strength on whatever you do will be needed.