problem aligning transparency for PCB solder mask

I'm making my 1st double sided board using Eagle CAD. So far I've managed to create the board design and transfer it to my copper using a laser printer and laminator, etch it and apply a tin plating. I'm now trying to apply a solder mask and I need some ideas. My board is small, about 1.5" x 1" so I don't have much room for error. I printed the solder mask layer from Eagle onto a transparency sheet, again using the laser printer. I have a very low end Brother which doesn't officially support printing to transparencies. In doing some research I found out if I tape it to a sheet of paper it will go through the printer. My problem is that even on the thin paper setting, which I thought would be the quickest speed so as not to overheat the paper is that the transparency material has a distinct bow. With it cut to my board size it is raised up about 1/16" in the middle so when I put the glass down it shifts. I tried taping each end but it still moved some. Has anyone run into this problem before and solved it? I'm anxious to get parts on my board to see if it works as designed..