Yeah I use PLA pretty much exclusively.
There is a little flex anyway, until you've mounted the PH to the top mount. This was intentional to keep weight low.
The PH is providing that stability on the top mount. Kind of the same concept on the bottom mount. I make sure to cut some adhesive around the corner pieces so they're stuck nicely to the top plate.
FWIW I use 3M VHB on all my builds.
I might tweak it a little today. If there's something specifc you need changed let me know and I"ll see what I can do.
Edit: The top mount should have a recess to allow the dampers to sit flush. It doesn't look like it came out in the print.