Hi Mike,
I have a die cutter for making medium format 3D slide mounts out of cardstock. It is sometimes useful to have cropped apertures, so I got a die made for 35 mm panoramic format (23 mm x 52 mm window.) Unfortunately my mounts are designed to be used in a handheld stereo slide viewer, so the dimensions are a bit different than your 7x7 cm projector. Here's a picture of the mount:
View attachment 217434
I tried cutting down the mount to make a single 7 cm x 7 cm mount, and this is what it looks like:
View attachment 217435
View attachment 217436
As you can see, there are a few issues:
(1) The window is not vertically centered .
(2) The top left left corner is slightly rounded
(3) The reverse side of the mount has a sliver of paper missing from the side of the mount (see bottom left side of the photo of the open, cut mount above.) This is because the aperture/window on the reverse side of the mount is 2 mm larger in each dimension (25 mm x 55 mm) than the front aperture. (This is needed so that "the stereo window" is defined precisely by the the front window. Even if there is some imprecision in the folding of the mount, the stereo window is not occluded by the window on reverse side. I'm not sure if I explained that very clearly, sorry.)
I've never projected medium format slides, so I don't know if these issues are cosmetic nuisances, or will cause more serious problems.
The material is heavy weight cardstock (110 lbs) and is advertised as acid and lignin free. There is no adhesive on the mounts. For my application (handheld stereo viewer) I use Scotch Magic tape which I understand is photo safe for mounting the slides and sealing the mount. I'm not sure what would be a suitable adhesive for a slide projector that I imagine may heat up the slide.
Anyway, I realize that what I have here is not exactly what you are looking for, but if you think they might be suitable, I can drop a few in the mail for you to check out and see if they will work. Just send me a PM with your address. I started off cutting these mounts by hand with an x-acto knife, but I quickly became tired of spending half an hour on a single mount!
Ian