I was hoping this post would lead to more discussion, but it appears that I should've just formed my answer in the form of a question.

So I'd like to add some extra strength to the elastic (bottom) portions of my overboot(ie)s. Any material suggestions?

If you don't feel like looking at that link above, here's a picture of the parts I'd like to reinforce. The elastic parts form the "T" along the sole of the shoe.

You might sew a strip of neoprene decking onto the wear surface of the elastic.

My gaiters have an instep strip of neoprene, and those suckers have been worn for 25 years without fail.

A heavy version of neoprene is what some snowshoe decks are made from.

--B.G.--