I had some parts on order for a Heated bed - Both the Alu Clad resistors and the Aluminium was quoted as two weeks delivery, so I used a rather scratched bit I already had to test a Heated bed.
MDF base, then a layer of Aluminium Foil (not shown)
Opal Polycarbonate 3mm sheet.
Silicone baking sheet (240 deg rated)
Then 3mm Aluminium sheet.
And because of not having my Resistors, I didn't want to use Nichrome Wire, so I went for a quick and dirty method others seem to have had some success with...
Sand the base with fine wet and dry.
Add a fine layer of heatsink compound and fix to Aluminium plate.
Wire it up and isolate.
For PLA It's getting 55 deg C-in the middle and about 50 deg C around the edges, may need a little more tweaking.
Without the glass and at maximum power I can get it up to 130deg C (120 edge) so it should just about be ok for ABS? - Hopefully I can test it tomorrow.
The MDF only gets slightly warm so it seems to be a reasonable use of materials, I would have liked to use PTFE Sheet, but maybe on the next one.
The very next day my Aluminium order turned up, so at least I can build a really nice one for the next machine...
Still waiting for my resistors to get here, but I'm really happy with the Iron for now.