said by Draiman:
I was just talking with a co-worker who suggested just using standard 1" x 3" strapping since it's cheap. I don't mind the labor but would that work?
If this were my attic this is what I would do.
Since you have a small access portal and all you want is crude subfloor for storage I'd use dimensional lumber rather then plywood. IMHO 1x3 strapping is too narrow. I'd check with your lumber yard to see what they have that is cheap but wider, 1x6s are ideal. Since this is not tongue and groove lumber each piece needs to be able to fully support whatever load is placed on it. A crappy 1x3 with a big knot in the middle of a joist span means your foot might go right through the board.
Using dimensional lumber rather then plywood also makes it easier to add wiring or plumbing later since you only have to pull up a few boards in critical locations rather then large sheets of plywood.