Hi jaswhite,
Post #89 here talks about the pros and cons of different types of mattress protectors and has some good examples of each.
The St Dormier is one of the suggestions mentioned there and it’s both thin (for a wool protector) and stretchy and would have less effect on a memory foam mattress than others that were thicker and less stretchy but by the very nature of having a thin wool layer it would affect the memory foam a little more than the other two types (the thinner membrane type and the stretch cotton type).
Phoenix