a beach ball would almost certainly pop. I'd buy a sitting workout ball, but one made a shorter person. something like this size:https://images.app.goo.gl/Dv5QGXrd6MfzQs5P8
make a bunch of (only mildly filled) water balloons and put them in the medicine ball, along with a bit of sand and a lighter material to max out the ball without breaking your spine. maybe an old, poofy coat that still has all the waterproof nylon intact. and some way to cut open/ reseal the ball (dunno how to go about this. A lot of duct tape would probably work though)
cut it open, put the sand on the bottom, layer on the balloons, put the fluffy material in the center, fill the rest with balloons. reseal. maybe add a bit of air to make it regain the round shape.
then just buy some pregnancy support garments (and yeah, probably a xxl leotard) and you're golden.
note: I have never done this before, but I've read a metric tonne of forum posts about the making of fake bellies, and this is a combination of all the posts that said their fake bellies worked amazingly.