Ruby lith tape is one answer, but a little fidly. Another option would be to use the black sticky backed plastic made for signwriting and graphics. This can be stuck onto the 10x8 sheet of glass then an aperture cut out in the correct position and the unwanted part peeled away. A small strip of actetate could then be taped on top of the plastic, along one side of the aperture, to act as a crude registration device (a la Paterson contact frame).

Having said this I'd also consider two further options which may or may not suit your intentions but might make things a little easier, if you're prepared to think laterally.
1) Make a same size print, using an enlarger and masking frame, thus avoiding all the problems you'll get regarding general fiddlyness and registration.
2) Make a contact on 5x7 paper, then dry mount it onto a larger sheet of board (or even unexposed 'fixed' photo paper) - not precisely the same thing but visually similar.