I was able to salvage the foundation blocks from my previous layout, so the first three photos are older ones that show how I built them back in 2018. I cut a piece of styrene, and started gluing 3-foot strips to that, spaced out to match the spacing of the tipple's support structure. These were all cut from sheet styrene of several thicknesses; basically, what I had leftover. The weights are there to make sure the strip doesn't move as I apply the Testors glue.
Here is the final construction of the styrene "collection". I kept adding layers until I reached the desired combined height of 14 scale inches.
After setting up a temporary system on my tablesaw, I carefully cut each of the 3-foot strips across the rows. Some pieces came off during the cutting (not enough glue had wicked in all the strips). Also, the speed of the blade caused some material that was being cut to melt and coagulate. It left the strips looking rough, but it actually went pretty smoothly. Easy enough to clean up with a file, which I wanted to do any way, to remove the sharp edges around the "concrete" blocks. This now left me with scale 3'x3' blocks that are 14" tall.
For the previous layout, I had already started laying the ties for the track, and so I applied the strips as they were shown in the previous photo, to the surface of the layout, i.e. in between the ties. For this new layout, I had that knowledge already, so I could prepare for it better. Also, removing those strips of foundation blocks damaged most of the in-between thin sheet of styrene, so I cut off the blocks themselves, and cleaned them up. A few of them needed to have their bottom layer repaired, but most were just fine. This saved quite a bit of time.
For this new layout, I cut two pieces of 6"x24" of sheet styrene, edge glued the ends, added tape to protect that weak connection, and trimmed the overall length to just a bit longer than what I need for the tipple's foundation. This sheet is the yellow sheet you see in the Tipple Design page.
I then spent time drawing out the intersecting lines to exactly determine where the foundation blocks should be glued. It is much easier to erase incorrectly-marked lines than it is to fix wrongly-glued-on foundation blocks. I quite literally quadruple-checked these dimensions over a period of two days.
With their spots clearly marked, it was pretty straight-forward to glue the foundation blocks down to the big sheet. This is all styrene-to-styrene surfaces, so MEK was used for that.
From having recently demolished my previous modular layout, I learned that gluing styrene to ceiling tile using Aleene's Tacky Glue worked wonderfully. The strips, shown in the third photo above, that I had so glued to the ceiling tile in my previous layout, were not easily removed. So, given that, I spread a thin coating of Aleene's Tacky Glue on the marked-off area on the ceiling tile "ground" of my new module, and glued the sheets to it. I put a heavy weight over the whole assembly and let that cure for many hours, just to be sure. The excess you see hanging over the open creek area, in the right foreground, will be trimmed off later.