I decided a flat wreath form. If you wrap a wreath in something full or fluffy then flat doesn't matter. Actually with a full medium sometimes flat is good so the wreath isn't too huge.
Cardboard is one of my favorite mediums lately. Mostly because its free and I seem to have an endless supply in my garage.
I have yet to have problems using cardboard. Pinterest has ideas about cheap wreath forms but in my experience they don't work.
Pool Noodles-- kink
Insulation tubing-- stretch over time.
Cardboard no complains, definitely not foam, but works well in its own right.