Matt, try to elinimate glass if possible, makes the dust "battle" a bit easier. On my old Opemus, I replaced the bottom glass with the black sheet of plastic of the same thickness. I cut the hole slightly bigger then 24x36 in the middle of this plastic. Then i took plastic Gepe slide mount and adjusted it with a knife and a file so negative passes thru easily, without any scratching. Then when my negative is align, I squeeze it just like when mounting a slide and put it in the carrier. It keeps the film very flat as well. I cut also little notches on the edge of this mount, so it can be prey open with a nail. I know, sounds like a trouble to go thru all this, but I like it a lot this way.
Metod