verify: yes, masonite is a brand name used generically for hardboard ( like we say " I need a bandaid " in North America when we want a bandage ).
So...here's the update on the baseboard materials;
I bought a sheet of hardboard ( shiny on one side, rough pattern on the other ) and I cut a base shape. since I'm using black house paint with sand for texture it has a lot of moisture in it and it soaks in...even with hardboard...so I got warping. Not massive warping, but enough to lift the edges and make it look like characters have to use a step to get onto terrain pieces. Aesthetics are very important to me...if it doesn't look good, it's not rewarding.
I used a sheet of Plexiglas to try that and it is Fantastic!
The best part is, Plexiglas comes shipped with paper to peel off on both sides. I left the paper on and painted over it ( thus I did not have to rough up the plexi surface to get tooth for paint. The pieces have remained absolutely flat on my gaming table!
I also thought " hey, if I get bored with the finish i've put on this mounting board I can always peel off the paper and start over ( then I'd have to rough up the glass surface...but meh! ).
so...Plexiglas = win!