Some more progress.
Given the gap i Had with the hill section I redid the ended to make sure there was a tighter fit when the sections are butted together to make a single hill.

Above is a shot of the sections with the new end pieces attached. I do apologize for the shop-safety misstep - I left the blade exposed of my box cutter - not the brightest of moves.

One benefit of redoing the ends was it gave me a chance to make the third level a lot bigger which will allow for better gaming situations. The previous version's third level was small (5" across) and only on one board so. Now the third tier is over 18" long and is worth "fighting over"

Plaster and plaster cloth to blend the new third tier into the rest of the board

Lastly covered with a mixture of 1 part white glue and 3 parts tile grout. This mixture provides a nice ground texture and dries rock hard. The only downside is that the addition of the white glue extends the setting process for up to 36 hours. From this angle, the new portions of the hill kind of look like a shark!
I had enough of the grout / glue mix to also cover 2 of the four remaining 2'x4' panels. The other two will be done after I get some more grout.

The current river bottom is just black styrene plastic. I'm debating if I should just paint it or add sad / gravel before adding the epoxy "water" - any suggestions from the forum on how to do a river would be greatly appreciated.

flat boards aren't the most exciting to do terrain wise but they do need to be done.