Another less expensive option for a less demanding floor covering is cheap linoleum, the kind from a discount flooring shop.
Flip it over , cut to size and paint the backside. Most inexpensive types will lay flat if given a chance to warm up and will actually take paint well .
Clear tape the...
A poor soul, down & dirty approach , depending on the size of windows , you could look at scaffold netting to cover the openings. You may find a benevolent company willing to donate pieces large enough to work with. Another option may be "screen door" material.