Nope, no spare trays. I had an old...forget the brand, but a projector I had in high school when I couldn't afford a Kodak, that used straight trays, but it's long gone or buried in my parents' basement in TN. I have a Medalist projector and some carousel trays, but I use those. I don't have a medium format projector. I think "super slides" where slides nearly 2" square that were shot on medium format and mounted in mounts with thin margins for projection in 35mm projectors. I've never seen one or used them, but I would get a medium format projector IF the film weren't going away. 6x6 or even 6x4.5 slides have to be awesome (6x7 even more so, but then the projector price and rarity goes up a great deal.)
Why do you say MF is "going away"?
I have a bunch of 6x6 slide mounts but I'm hesitant to cut the 6x7 images since they all look better in that ratio than in 6x6