It is very easy to make a gasket for the Rolmonica.  You can cut
the square holes out of any good quality sponge material, such as
1/8-inch-thick Neoprene or polyethylene.  Press the material against
the harmonica or the case to get an impression as a cutting guide, and
use a sharp X-Acto knife blade or a small scalpel to do the cutting.

You can also use a self adhesive variety, stick it on and cut it in
place using the case or harmonica as a guide but be careful to not
damage the harmonica or case with the knife.

Do not use urethane unless it is of a known long lasting quality as
it may turn to goo in a few years.

Spencer Chase
Garberville, Calif.
http://www.spencerserolls.com/