I just ordered a 2" memory foam mattress topper with the intent of putting it on top of the existing mattress. I can't speak to its effectiveness yet, but it sounds like that's what a lot of people do.
Glad to hear that. I was hoping to do this when my tent comes in mid April but I didn't know if this would interfere with it closing or from keeping bedding inside.
My home mattress was a bit too firm for me and an inexpensive 2" memory foam topper made it just right.