We picked up our new to us Scamp 16 last fall and we're also looking for a mattress solution.  I think we're going to go with something like a 3" or 4" latex topper and just leave the bed set up all the time.  In reading many posts on bed size, it seems like there is some variability in measurements being taken.  But since we're going to cut ours to fit, we're not going to sweat that too much.  Anyway, various owners of the Scamp 16 report a width of 44" or 45" and an length that varies from 74" to 78".  We've seen a full size latex topper listed at 53" x 75", so that seems like a good place to start.  Many folks suggest using an electric carving knife to trim the topper after carefull scribing the curves and actual width.