Without pressure casting, bubbles are probably going to end up in the cast cause you're just sloshing around the resin. I would imagine, if you cutup another resin rotocast piece, you'd find some bubbles as well, but since it's not clear, it's not an issue.
I wonder what would happen if you had a rotocast that could fit into a pressure pot.
Also since it dosen't have to be clear, just frosted, you could always pour your mold open to get a thin layer like a rotocast, then join the two halves together with more resin along the seam, maybe add some cabisil to make it pasty.
don't know if you would need to vacum degass it, could help though.
Another thing I just thought of is that if your trying to get thin walls and don't want to rotocast or can't inside a pressure pot, you could make a void that can be removed. This would also require joining the peice after both sides are cast.