Proper cardboard for slide mounts?

Can anyone suggest the best type of cardboard used to mount color slides? The type Kodak used is not thick--about the thickness of regular chipboard--but much stiffer. I have a die cut for manufacturing my own Super Slide cardboard mounts on a letterpress. These are the 4x4 cm format mounts for 127 film or for cropping down 120 film to run alongside 35mm slides in a regular Carousel projector.

I have a good adhesive system for mounting and already made some of these from regular chipboard. They work pretty good--but I'd like something stiffer.

Gepe no longer makes glass Super Slide mounts and new old stock ones go for a fortune on ebay--so I really want to be able to make my own cardboard ones.

Advice appreciated.

I don't have much detail at hand, but I would take a look at "tag" board or "Bristol" board. Some art supply places carry Bristol board in two surfaces and 2, 3 and 4 ply, it might be worth a look.

Made some tonight with Bristol Board and dry mount tissue. I die cut the tissue to size same as one side of the fold-over mount and positioned it with a tack iron on the corners before mounting the slides (base side up facing the tissue). Sealed with the same tack iron pressing down all four sides from the outside. . Seems to work quite well much like the old Kodak Ready-Mounts.

Could you use 35mm mounts, with the opening enlarged?

You may want to check the Reflecta mounts available from here -> http://www.scandig.com/slide-photography/slide-mounts/reflecta-cs-slide-mounts-200.html

These are re-usable and have been using them for years. Also they do not get jammed like the typical cardboard mounts.

Yes I like the idea of starting with 35mm heat seal mounts and cutting them larger. I tried this and they seal within about 2 seconds each in my Seary Slide Mount Press. Aside from projection, this also makes a convenient way for scanning 127 film since there are no templates or holders available for unmounted 45mm film with computer scanners. Thanks.