Flattening paper ain't hard, but it takes some space and some equipment, and two weeks.
Go to
www.talasonline or wherever and order some Hollytex (I like # 3257). This is placed over and under the sheet that is being worked on to help prevent media loss. Also order some blotter paper -bigger and thicker is better.
You can make a temporary humidity chamber the size you need with 2 x 4 's and polyethylene sheeting. Make some kind of frame (you know how to do that) that's big enough, and stretch fiberglass window screening on it really really tightly, so that it does not sag when you put a piece of paper on. Wet a towel or towels (ordinary tap water is generally OK - if you leave a bucket of it sitting out over night, any chlorine will evaporate out), and lay these in the bottom of the humidity chamber, put the screen in, screen side up, a sheet of Hollytex (about 2" bigger on all sides than the thing being flattened), then put the thing to be flattened on top of the Hollytex. Cover the whole thing with that scratched acrylic sheet you can't use, and leave it there for several hours.
Have a stack of at least three sheets of blotter ready, and another sheet of Hollytex (the same size as the first). Lift the thing to be flattened by the Hollytex, place it on the blotters, cover it with Hollytex, then put three more sheets of blotter on top. Put a sheet of thick acrylic, laminate-covered board, or something else just about dead flat on top of this, then weight is all down as evenly as you can with bricks or books or barbell weights, or whatever. Leave it there for two whole weeks. Then remove it, and it is going to be as flat as it's going to get.
The instructions make it sound a lot more complicated than it is.
Make sure you get all the paste from the hinges off, or there will probably be some buckling there.
I attached a snapshot of a print in our admittedly more permanent humidity chamber, which is a giant sink. I don't have Hollytex under the sheet (it's important when the paper is fragile), and we use so-called egg-crate (from older-style florescent lighting fixtures) instead of fiberglass window screen, but the idea is the same.
05.
www.ica-artconservation.org