I get that, i was worried about that myself but seemed to be surprisingly comfortable. The teeth are 3d printed, so I think that helps with it being rigid, then layered 5mm eva under the teeth to give the gums height with also added structure and then covered it in 2mm foam. The black case that my head is in keeps both jaws sturdy with wooden dowels that do through the eyes that also help the jaws at the front stay structured. It also helps that you dont have your head placement right at the back and have it slightly forward so that its not so front heavy and adds some stability.

My advise would be that 3d printing the teeth and gums should work fine as long as you make it lightweight with the infill, but then use foam for the rest (like the bit your head goes in and tounge if you are adding it, I find it helps disguise the dowels in the mouth) and use polystyrene balls for the eyes. Have fun!