Since I have my own auto upholstery business, I have always made my own amp covers, I've made some padded ones using cordura nylon fabric. And just basic vinyl covers using some good heavy vinyl. Not too hard if you can sew and have a machine that will work, and I do. I'm sure Studio Slips are great and they are in business to make money so I can understand why they cost what they do, I'm sure that is a limited market selling amp covers, and they probably make other items too.
I'm amazed at how many amps I've bought used over the years that would have been in much better shape IF the owner had just used even a cheap basic cover. Like this one my most recent purchase
Which I will clean up and make a cover for.