I have the sliding bow seat in mine and it helps with the problem, mainly because the smaller person will normally slide forward to adjust the trim and therefore be closer to the floatation chamber.  One thing to look for is the fact that if they put their feet against the chamber and push very much sometimes if there is a weaker spot in the chamber edge where it is sealed... it will come loose!  That happened in mine and I had to re-epoxy it to seal it.  The best thing to do I think would be to use the foam or Styrofoam blocks cut to fit and put it with velcro tabs for easy staying and removal.

Bruce