Thanks for all your suggestions.
Unfortunately none of them seem to be available here in Norway. I suppose they're commonly sold in bulk for industry-use and don't pop up as something consumers generally can buy.
I originally bought an acrylic plastic display stand for this, but decided against using it because I don't know how it handles heat (fire hazard?) and if it'll emit dangerous chemical fumes when heated, which I definitely don't want.
An idea struck me though: how about those silicone baking mats? They're made to handle high temperatures no doubt, and silicone should insulate electrically as well.
Even cheaper: baking paper. Equally well for high temperatures, but would it do for constant heat, or will they break up or something? I'd place several sheets on top of each other for suitable electrical insulation.
I don't know how hot a computer power supply PCB would become from underneath, but they do at least give off some heat as long as they're turned on, so the material should be able to handle long-term heat, even if it won't be very high.