I'm working on a project for school in which I slice up pieces of 35mm motion picture film leader/scraps which resemble (mostly) famous abstract paintings (Rothko, Barnett Newman, etc.) and put them in slide mounts so that I can project them onto the walls of a gallery, giant sized using a carousel projector. The problem is that obviously the way I want to compose each slide does not always fit perfectly within the (approximately) 36mm x 24mm frame of the slide mounts. I therefore need some way to mask off certain parts of the slide. This is a piece of fine art, which I hope to add to my portfolio etc. therefore longevity is extremely important. Obviously most types of tape are off limits for archival purposes, heat sensitivity, etc. (like electrical tape and black hockey tape). My professor insists that there is a specific tape dedicated to this purpose but I'm thinking it's a thing of the past after unsuccessfully searching B&H's and Printfile's websites. I figured APUG would probably know best. I'm thinking if such a thing ever existed it was probably just a re-branded aluminium foil tape. Recommendations/advice would be much appreciated, please and thank you!

I've attached some pictures to explain myself if this^^^(above) was not clear enough (I want to turn image1 into image2).