Whats the best way to make the inside of a topper look good?

I used the gray carpet they sell at Lowe's, HD, etc. on my last shell. It's a little heavier than speaker cloth and I had no problems with it as a floor covering on my sleeping platform before I did the shell i.e. easy cleanup, no mildew, etc. It's easier when the shell is off but doable with it on I'm sure. You can kind of test fit it and trim up some of the excess to make it easier to work with, just be sure to leave excess until after you've fit everything and it's dried. Make sure you use the heavy duty adhesive and once it's stuck then trim it with a sharp razor knife.