We added foam mattresses to our bunks- what was there originally was about 2 1/2" thick and didn't support adults whatsoever.
I ordered 4" foam from www.foambymail.com
in custom sizes. I was comfortable with just the foam laying on the floor. We don't have height issues, so I just put them under the original mattresses.
I'm also confused why you're putting the topper under the mattress?