insulation

got a problem - i'm installing 15 inch wide insulation into a flat roof from underneath. I know I need to keep an inch gap on top of the insulation, that stops me from tacking/stapling the insulation up there. However, my joists are 20 inches apart at center.

So, I need to cut an extra strip to supliment the 15 inch wide - but how do i get it to all stay up there until i hand the ceiling. I'm afraid to ask if packing tape is okay?!