Personally I prefer Foamy Lizards cover for my Xbox 1 and if they were still in production I'd grab one for my 360E. The benefits of Foamy Lizard over the competition is 1) their covers are form fitting to the devices they're made for and 2) they come precut to account for the cables protruding from the back of the console. Again, not sponsored.

Foamy Lizards line of products aren't designed for retro consoles however so here's a few other places to find what you need:

Their console covers aren't quite as snug on the system which means there's a higher chance of dust entering the device but it would have to be kicked up with a breeze. The only way for it to enter is from underneath. This isn't a major issue by any means and they still provide a quality product. I do have 2 issues with their covers and they are:

1) they put their brand name on the front of the cover. This is a personal discrepancy, it does not hinder the function in anyway. It's just not as uniform as a solid black cover. Plus, I don't get paid for displaying their cover so I could do without the writing. However, I dislike dust even more than this so I can live with it. 2) their covers

don't account for cables which means unless you cut the cover you'll have to plug/unplug your wires every time you want to play or cover the device. Again, not a major issue. Just take some scissors and cut the fabric out where your cables plugin.