Been pretty happy with one of these cheapo Amazon covers. Soft and thin, gathered at the bottom so it doesn’t hit the floor. Does the job keeping dust off. Don’t use one on the M4 so I don’t know what size would fit.
Cosmos 10323 Indoor Car Cover Elastic Breathable Dustproof Full Super Soft Protection 130gsm Fabric in Medium, Black
https://a.co/d/2fuHDQ3