Vermiculite plates are usually used to insulate fireplaces, so I bought some and cut them to size for my oven, using a circular saw. One of them has all the holes for tubes, thermocouples and heater connections:

As you can see this stuff is a bit delicate and I might need to surround it with sheet metal or something similar.

A piece of perforated metal got saved from being scrapped and now serves as a base plate. This is how the oven will look like from behind, more or less:

The lid and electronics still need to be done.