I have used block foam to support big stuff for decades. When I was in the marine business, we used it under boats when they were stored on the ground. Both fiberglass and pontoon boats and often over the winter and sometimes for years. Have done it thousands of times, literally, with never a problem. We used the same blocks year after year. And many pontoon boat companies use closed cell foam IN their logs, which, at the time, was where we got the foam for our boat storage.

Today, I still have a few lying around that I got from Rural King that I use under the Cuda occasionally as well as for other jobs. I remember getting them from RR and that they didn't cost much, but it's been long enough ago that I don't remember just how much. It is not something that they keep for retail, but will sell. I get my wrapping plastic there, too. It's not stocked for retail either, but they will sell it from their supplies too if asked.

And yes, there certainly are different strengths of foam. That link I posted earlier has, at least, some of that info.