Have actually done this with good results. I used piece of black paper (11x14) cut it off folded so that a piece of 8x10 paper would fit with a window cut into it for the 5x7 negative, use some rubylith tape on the corners to hold the negative in place (on the inside) and put the whole thing in one of the Preimer contact printers. Worked perfect....was able to print 12 prints in a session without any problem. Just placed the 8x10 paper into the fold and lined up the top...in my case I had the window cut off center so that there was more 'white space' at the bottom.
Hope this helps.