Confusing directions. But I have gotten it to work. I changed some things to make it do so because I don't want duplicates in my text files.
1. You have this set as a foundation and not a deck, therefore you cannot place on top of a foundation or other deck. Of course I want it to function as a deck.
Updated script to make it a deck:
50_WhiteDeck.txtcatalogTextResource 50_WhiteDeck 138
buildTipIndices 5 5
deck 50
wallStyle 200
floorPattern "deckDarkRedwood"
supportObject 0xAA75A097
supportObjectUpperLevel 0x9EDE28AE
catalogTextIndices 0 1
2. To make rounded corners pool corners work and custom foundations.
Move the scriptorium line from the base game deckfoundations.txt to your latest ep's deckfoundations.txt. Seems very redundant to take the scripts from later ep's and add them to the base game, thereby doubling the scripts.
The remainder of the scriptorium I left in base game TSData since they work.
I have not extensively tested this, but I have spent about a half hour building with this. I only have a couple of custom modular stairs, and I have not tested this with Numenor's fences. Mostly since I hesitate to use anything that cannot be packaged with a lot.