The local Home Depot doesn't stock anything other than 14.5" wide styrofoam.
And no 22.5" wide double-faced fiberglass.
Looks like my three choices are:
1) Cut the styrofoam I have to 22.5" lengths and install "sideways." No protective gear required. Messy when cutting. Available locally. R is only 3/sheet (but there's room for up to 5 layers).
2) Blow in some cellulose. Very messy when installing. Face mask and goggles required in a hot attic. Will be renting equipment, so some time constraints. R 10-15.
3) Use single-faced fiberglass. Protective gear required in a hot environment. I hate fiberglass - I've been exposed several times and it's not fun. R about 15.
Unless someone has a better idea* for insulating my ceiling, I'll try cutting the styrofoam first and see how it goes.
* I would prefer something that doesn't require a lot of protective gear (it gets hot in a S AR attic), can be purchased locally (the biggest problem), and is reasonably cheap. Greater R values are better but it's just a garage.