Let me make sure I understand your question. Do you want to process 8x10 sheets, or do you want to process four 4x5 sheets in an 8x10 tray?
If you want to process a single 8x10 sheet, then you don't neet a slosh tray - any 8x10 tray will do.
I made a slosh tray to process six 4x5 sheets in an 11x14 tray. I've also thought about making a slosher to do four 4x5 sheets in a slightly oversized 8x10 tray - but I don't have the oversized trays. Right now, what I need is to use the stuff in my darkroom, not add more stuff that I don't use very often.
There are two basic designs for slosh trays. The DIY version I made uses short strips of plastic to separate the sections of the tray. These are long enough to keep the film from migrating between sections, and yet short enough that developing fluids can move freely between sections without creating eddy patterns that result in uneven development. The other design uses nylon bolts to perform the same function. Commercial slosh trays look more like the plastic strip DIY designs, only they tend to be molded from solid plastic.