OK, I've made a flat mound Russwood/Red Elm Park.
The zip file is here:
Google Drive Link to zip file with relevant OOTP pieces. If you want the Sketchup file I made it from, it's here:
Sketchup file for Russwood/Red Elm.
I gave the files in the zip file the same names as the version with a mound, mostly because I forgot to change it. :| On the other hand, it should mean that they can be dropped in where the other version is (while OOTP isn't running) without needing to repoint OOTP at the new files.
I haven't played a game with this version of the park, but I did load it in OOTP in place of my previous version, go to the 3D model pane and pan around/zoom in, and it all looks right (see screenshot). Hopefully this works for you, if not we may need to think some more about details.