Stump, mine are glued in with silicon, although I'm sure there are better ways.
It will be tough to glue baffles into a rubbermade sump anyway because of the sides bowing out. I would try it w/o baffles and see how it works...
If you needed to you could get a smaller rubbermade to house the return pump. Put some holes toward the bottom so the water enters the bottom... that may work????
Good Luck,