I didn't find where the plastic parts came from, but I'm assuming they are made from fuse holders. I did see the comment on them heating up, and that would be my primary concern. The plastic didn't look very high-quality to me, and if a connection starts to heat up due to a not quite perfect connection, and that starts to soften the plastic, it's going to make the connection worse, which is likely to snowball.
The plastic should only be acting as a stiffener, so there's not need to put plastic between any of the metal parts. If I were doing that, I would just bolt it all together, metal to metal, and 3D print a frame to go under it / over it. If you imagine the bolt heads on the bottom fitting into holes in a solid plastic base, and then some screw clips holding it in place, that's all you really need.
