Clear film encapsulation can be done using Mylar-D, Melinex 516, or similar polyester film. Invisimount, which is polypropylene, can be used also, but it is less clear, less rigid, and tends to stretch/deform a little, which makes it more difficult to create a perfectly flat mount. Both polyester and polypropylene are suitable for preservation framing, so long as they do not have chemically-invasive or unstable coatings.
If you make your own encapsulation mount, do not use ATG or any other carrier-less tape. Use a good quality double-sided tape with a polyester (non-stretching) carrier. I like to use ASAP Tapes #ASA600 fillet tape or 3M #889, but 3M #415 is more popular, and other fillet tapes would do the job. Be sure to burnish thoroughly for a strong bond.