I own a jumbo 3D sponge bunny, but find it's quite a handful. Does anyone have advice on good ways to hollow it out?

I cut sponge with an electric carving knife.

I'd just impale it in an inconspicuous spot and reach inside and pick out the sponge a piece at a time. You should be able to feel how close to the surface you get so you don't cause damage to the outside.

I decided to give it a try and hope for the best.
It most went pretty well. I managed to take out about 10%-20%. It would be possible to take out a little more, but I came close to ruining it as it is.

It hasn't reduced the mass as much as I would have hoped, but I suppose that's only to be expected as the inside is a lot smaller than the outside.

I don't remember where I read it (perhaps Frank Garcia's Encyclopedia of Sponge Ball Magic) but there is a technique where you pinch the sponge between the first finger and thumb of both hands and basically rip the insides up as you pull the fingers apart. This shreds the inside of the sponge but does not need a hole. You repeat the procedure throughout the sponge. It makes a regular sponge much more compressible.

I saw someone mention that previously on the Café - as I understand it, it makes the sponge larger. Although I'd definitely try it in the case of a sponge ball, I don't know what the results would be like on a bunny.

Can you try cutting 2 bunny shapes on thin foam, then making a long ribbon that can be glued in between? that would make a perfect pocket in a foam shape. so it would be like a hat box, except instead of a circle it would be a bunny shape on top and bottom...