As an experiment, I made some Top Bar (frames ?) by nailing some plastic
foundation, itself quite rugged, into split top bars.  I was told that the
bees would make a mess along their bottoms.  The bees did not make a mess
in the brood chamber, but did attach bottoms and sides where they make
honey.  After a couple scrapings-off of the mess, the bees gave up and
neatly rounded the comb bottoms extending very slightly below the  plastic
foundation.  Since I do not extract, these top bar combs would work here,
but are probably not worth the bother of the initial scrapings, especially
if the bees again try attachment  every year. These combs are compatible
with and  mingled with standard medium frames.  The bees draw waxed plastic
fine as long as plenty of food is coming in.  I haven't tried the unwaxed.

Bill Morong