Have just got a high impact polystyrene vac-form test from a milled mold. The panel fits in the box (currently in the fabrication workshop).
V2 Battery Backbox Mold: The first test mold had a couple of issues.
First, is that the material used was not appropriate, foam melts under heat after a couple of mold forms, so I need to remake it from a material that is less sensitive to heat. MDF will work well from what I have seen.
Secondly, the mold needs to have wider lips to make the edge cut neater. There also needs to be additional material added to the top because the new design for the box has the bottom panel welded onto the underside so the panel drops 1.5-2.0mm. Best to add 10mm and cut it after. I think an extra 10mm extra lip side and top will be appropriate.
New drawing for the second version are below.