Tiny version of the mold printed 1/3 size in PLA quick 'n dirty style, with low infill and 0.3mm layers; took about four hours. Taped top and bottom halves together with electrical tape first, did a heavy application of canola oil spray and then taped the halves together. Just let the gelatin cool enough to not deform the PLA and dunked the whole leaky mold in a teacup of it. Set overnight in the fridge and the brain came out of the mold pretty good, though it was obvious which bits of the mold I had oriented properly on the plate and/or cleaned out all of the support structure on (oops).
Gelatin recipe: halved the water in the recipe for Knox Blox (1 cup water, 1 pkt gelatin) and subbed 2 tbsp-ish sweetened condensed milk for 2 tbsp of the cold water. Came out quite solid, but if you want something even more solid you'll want agar agar or maybe konnyaku.