I don't mount slides as the lab does on the rare occasions that I use transparency film but I recommend using plastic mounts if available. Years ago Polaroid had slide films and part of the do it yourself system was a 35mm illuminated slide mounter. It was battery operated and had a magnifying glass and a built in cutter. They had snap together plastic slide mounts and it was a neat little system. I still have one and once in a while use it. You might be able to find one or something like it searching the net.