If you need a cover for the rotors when cleaning your bike because you’re worried about getting the cleaning chemically stuff you’re using from contaminating them, the problem is the cleaning chemically stuff.
1) Hose off the worst of the crud. There’s a strong chance you can skip to step four from here
2) If you want a thorough clean, use a spray bottle with water/washing up liquid mix in it, spray all over (avoiding rotors, obvs, but it doesn’t mater too much), rub down with a cloth/brush
3) Rinse with hose
4) Quick, small, careful squirt of WD40/GT85 on mech pivots and jockey wheels. Use the straw thing so it’s only going where it’s supposed to.
Done.