The original description doesn't say this is a cheap vacation souvenir, and I've never seen one so large. Most of the souvenir papyrus pieces I've framed have been 16x20 or smaller. For all we know, it may be "real" art of significant value, whether monetary or sentimental. Phil, what is the value of this piece, anyway?
Custom framed things have an average useful life exceeding three decades, in my humble estimation. Of course, dorm room posters may be destroyed in the first semester, and many framed heirlooms last more than a hundred years. Parents and grandparents still display framed items that were hanging when their grown children were in grade school. Fifty-year-old frames are common. We develop emotional attachments to the images we see every day, so we tend to keep them around.
We do a lot of re-framing for old pieces in good condition, which simply need color & style updates. And what's the primary reason for discarding them instead of updating the framing? Fading.
Regardless of value, anything custom framed deserves a design that would last at least thirty years, in normal ambient conditions and with normal care & caution by the owner.
Pressing the papyrus between sheets of anything would foster foxing & mildew. Condensation is a problem, as Kit points out, unless there's a generous air gap between the sheets. The air gap prevents abrasion and other damage from direct contact, as well, and allows it to acclimate to environmental changes, as Hugh points out.
Hugh's pass-through hinge idea works great, but use plenty of hinges. Or, a similar look may be created by hinging over the top edge of an undersized alphacellulose backing board, invisible behind the art, which may be glued to the acrylic background.
I wouldn't use any permanent or non-reversible adhesive, such as silicone. There are plenty of better alternatives, of similar cost & complexity.