I want to get some neoprene shorts and try filling them up with slime/mess and wear them under my pants. Has anyone tried this, would they stop any leaking?

It is waterproof, but that only stops the mess leaking on the horizontal axis. You've still got a hole at the top and two at the bottom where fluid can be forced through as you move around, especially if you put pressure on it by sitting down. If the shorts were tight enough to prevent that, they'd also cut off your bloodstream.

The material is waterproof, but how waterproof the whole garment is depends how they're made. The better stuff is glued edge to edge and blind-stitched from the inside, and some cheaper wetsuits are stitched through, which might have some leakage.

A pair of plastic pants would do the trick as they have elastic around the legs for a reasonably snug fit. They also feel rather nice against the skin especially when filled with some slippery substance.

I have some plastic pants, they're good but they ended up being too loose to be able to fill completely. I want to feel submerged with the slop filled up to my waist so I'm looking into different materials.

I ended up ordering some good quality neoprene shorts with a high waist, I'll try taping up the leg holes and I'll let the thread know how it goes when they arrive!