Hi, i have made a couple of fleece covers with a double layer of regular fleece and found them to be Ok for using at home with fairly frequent changing and my son is not even a very heavy wetter. They feel quite dry when the prefold is wet but they let a fair amount of moisture through to the clothing which makes them no use when out and about. I even bought some 300 ct MM fleece thinking that stuff would perform better but there was no difference whatsoever.
Bottom line. From my experience MM and double layer regular fleece perform about the same way and are not really good when you are out or your baby will be sitting in a car seat or something. I have decided to stick with PUL myself after having a baby in a shopping cart leave a wet patch behind 'cause he was in a MM fleece cover.
I have had success with a two layer microfleece cover. I also only used it around the house but for that purpose, it worked great. I should add that my son was a toddler when I made this, so he wasn't sitting all the time like an infant would be--the stage of your baby might affect if you want to try this.