Trouble with Terrain

I currently have 4 2'x2' foam boards to use as a playing surface. They were originally created for Necromunda, and I simply textured and painted the bottom side as grass/dirt. I will soon need a 4'x6' area and at the very least will need to make 2 more 2'x2' boards.

However, I have grown to not like these boards. I sandwiched 2 1" thick insulation boards together, allowing for an "embedded" sewage/sludge river on the Necromunda side. Stacked up these make an 8"x2'x2' cube, and will be 4" thicker with the addition of 2 more boards. This is not easy to store nor to transport from garage to house.

So I have a plan to make a new (more) modular surface, using foam core as the base, with 1" foam board added for depth. I decided on 10"x20" tiles instead of 12"x24" tiles for 2 reasons:

1. I can get more tiles out per sheet of foam core, and a cleaner cut edge on the foam board.
2. It will be much easier to box and store

The trade off is having a "non-standard" board size. For small 40K games and Necromunda it will be either a 40"x40" or 50"x50" area, and for standard 40k games is will be between 40"x70" and 50"x80". Im not really sure of the impact, a few inches either way would make, but the closesest size would be 50"x70".

It will cost me $24 in foam core and $13 in foam board to make all of the tiles above. (Paint and sand extra). Pretty reasonable price, and a a very modular surface.

What are your thoughts?

-- edit
I picked up the foam core today (the art store even trimmed them to size for free!!) I have enough foam board on hand to complete this project, but will need more sand and paint. I also mocked up a 50"x70" table using my current hill pieces.