In most towns there's someone who cuts foam to size. 30 years ago in Corvallis, Oregon I went to The Foam Man
The Foam Man - Mattresses - Corvallis, OR - Reviews - Photos - Yelp
for a piece of foam to replace my VW Westfalia mattress. He had samples of different firmness for you to try out. We chose the second firmest and he cut a piece in 30 seconds or so and we went out with this light-as-a-feather mattress rolled up and tied. It was so comfortable and we slept so well in the RV that we threw out the mattress in our bedroom and had a piece of foam cut queen size to fit. Thirty years later I am still sleeping on the same type of foam.
Futons are the third most uncomfortable thing I have slept on, after a water bed and a hammock. Oh, except for the one night when I was a teenager, after a terrible party, when I slept sideways across a steel bath, but let's not go into that.