Initially downstairs I added a separate memory foam topper (5mm from memory) which improved comfort but was extra to roll up and pack up and so on, which was annoying. So I tried something a little different.
The cover for the VW Comfort mattress unzips, and you can remove the foam. If I recall correctly, they sandwich a harder foam and a memory foam together to combine strength and softness. I wanted more softness, so cut up my topper foam used that in place of the factory memory foam layer. Made it too thick overall, so reduced the firmer foam thickness. Possibly too tight now in the cover with the stitching under a bit of tension. But is still folds in three. And is softer than before.
Spent too much money on foam and glue, probably stressing the stitching, but a happier compromise. Would I recommend it - probably not. But you can...