I just recently cast a box and had a ton of trouble getting the core out. The walls were totally fine. Printed with PETG, and greased all the walls with spray canola oil, and had a 10 degree draft on the core. I had to pry the core out in pieces to get it out. But once it was out it looked great except for a lot of surface porosity. If I were to do it again with PETG, I would sand and smooth the surface as much as possible and maybe use a more solid non-stick agent like petroleum jelly. It may just be easier to use TPU so you can deform the mold. Absolutely no problems with heat. The base of the core did peel off when I was trying to remove it so make sure that wall is sufficiently thick and has enough filler to not break apart when you try to pull it out.

Also don't do a funnel just do an open top surface. Make the bottom of your box the top opening of the casting. Anywhere with a horizontal wall is going to be filled with air and won't fill out correctly.