ok... lol.. sorry
Manila Folder = Card Stock (in most cases)
If card Stock works, I would just assume a Manila folder would work, but wouldn't it just be easier to print directly onto a piece of card stock.
Drooping can be caused by applying too much resin, there by saturating the cardstock and weighing it down, causing the "droop".
I increased the number of layers of resin while making each layer thinner, lightly sanding between coats so that the new layer properly bonds to the previous. Try applying several very thin layers, rather than one thick layer.... I got this experience from sheeting wings of Airplanes that I build with what is essentially tissue paper and a doping compound. Tissue paper is very thin and flimsy, but if you apply each layer thin enough, you can get the desired stiffness by applying more, thin layers... Ok if this is confusing...
Think of it like glue two sheets of paper together... when you put a large drop of glue on the paper it saturates and wrinkles... where if you use smaller ammounts (but enough to adhere the surfaces together) you avoid wrinkles. Same principle.
Once a few layers of resin have been applied, it will seal the card stock, and you can begin to increase the thickness of each layer.