I put one of these on it. Cheap solution but its really comfy. I was super impressed at how nice it is. I sleep great on it now.
The cover is a bit too big because, like you said, the mattress is a bit undersized. So I just tuck it in around the bottom and it works fine.
Also this thing is kind of a pain to get the foam out of to wash it. So instead I leave it on the mattress for the whole season. I put a normal mattress pad over it. I carry two normal mattress pads so I can swap them and wash them whenever I feel the need.