You could alsways use sheet of the pick foam board for insulation under hte floors and just cut it in widths for a tight press fit. I like the foam board better than fiberglass batts anyway. You might also get a radiant barrier material and apply that first under the floor, that will keep more heat in as well. It's also a vapor barrier... so is the foam board is taped air tight at the seams.
Another option is spray foam... which is probably the best option since it seals air, vapro and has the best insulaiton value per inch . Just 1-1/2-2" of closed cell applied between the jostis and over the joists, would make a HUGE difference. YOu could also do 1" pink foam board first, then spray foam over it. SInce it's a relatively small area, you could get a DIY spray foam kit.
Remember fiberglass doesn't stop air movement. Worse, if there is air movement, it's R value drops... more like falls off a cliff. Its' also drops if it gets damp. Personally I hate fiberglass and cellulose. Spray foam or foam board is the only way to go.