I would try attaching one or two bungees laterally, across the raft container, from one grab rail to the other. They might prevent the sheet from snagging on the bottom of the cradle, guiding the sheet up and over the container as you tack.

 

Hard to tell how much space there is between the mast and the raft, but if you could either tie a line(s) or use bungees from the mast to the forward part of the raft (around the back of the mast and over the top of the raft at the two front corners) that would help to keep the sheet from getting behind it. Amazing how the sheets will find anything and everything to get snagged on.