I bought a pack of 6x7 heat-activated slide mounts. How do you get a slide into one of these and sealed properly without any buckling? I tried using a clothes iron and the problem I had was that the pressure of the iron on the side of the mount would distort it a bit, which meant that once all the edges were sealed, the whole thing was no longer flat and the slide was certainly not under tension; it had probably 1.5mm of unflatness to it which would make it impossible to focus for projection.
Using the iron flat across the whole slide (with a sacrificial slide in there) is no good because the iron will heat & shrink the slide.
I know one can buy a special slide-mounter but I've never ever seen one for 85mm slides. Is there some hacky DIY trick I'm missing?