2nd Gen Mattress Topper

I have a 09 6ft bed and im building a bed platform for it, but I'm trying to find a mattress that fits it almost perfectly. Was seeing if anyone have any recommendations

 

I sleep on an area bed rug on top of a full bed rug on the floor. I had a wooden platform in another truck but at the end of the day it’s just better without it (for me, anyway).

 

I got this mattress from Ikea and I've been happy with it! Full size fits perfect. https://www.ikea.com/us/en/p/morgedal-foam-mattress-firm-dark-gray-80277382/#content

 

not a mattress but the last time I did something like this, was in a 5ft bed and cut it diagonally to be longer
using 4" thick foam from a milsurp store. It was comfortable. Threw a blanket on top. After researching, it seemed the most popular choice was 4".
They did have others available, 1" 2" 3" and 4"

thought about maybe making a zip-up protective cover for it but the blanket worked fine.

if you do use foam like that, it can come in a long length, or have multiple pieces cut, combined, and potentially glued, to achieve desired length

an actual topper may be cheaper though. I imagine they come with a cover. Internals depending on model and what it's made out of. If you end up having to trim it.
Not sure what modifying the cover would entail, if that's something that can be dropped off at a sewing place to have changed.