There are several things to look for:
Thickness of the foam,
Density of the foam,
Quality of the vinyl,
Quality of the vapor barrier,
Reinforcement of critical spots like the hinge and handle areas.
I have sold covers from many makers over the years, but I recommend Doc - RHTubs.com - I find them to have all of these things nailed down. They are a bit wrinkly-looking, and at first I was put off by that. But after thinking it over, and talking to Robert and Andy, I realized that the thicker vapor barrier they use is simply going to be a bit wrinkly, and it is better than some sort of heat-shrink film which is likely to fail sooner.
Also, the vinyl tends to shrink over time, so starting a bit looser ends up being much better in the long run.