Okay. So.....
When I filled the tank initially, there wasn't as much weight on the side aluminium strut. Essentially that meant that a lot of the weight was sitting on the 4 corners - I wasn't happy with that.
So I ordered 2 more Fluval leveling matts. They were like 20 bucks on amazon or something. NBD.
So what I ended up doing was I cut two strips off the old mat, one for each side. Those strips of matt sit between the HDPE side shims and the aluminum frame.
Then I took one of the new matts and placed it on top of the aluminum frame to sit between said frame and the mdf base.
My thinking is that this helps to make up for slight variations in the contact patch between all the different layers.
Finally I took the other new matt and placed it on top of the mdf as Fluval intends for the tank to sit directly on a matt.
So it goes:
Tank
Matt
MDF
Matt
Alum Frame
Matt
HDPE SHIM
Desk
I didn't place matt between the hdpe and the desk surface because I like the way I can actually shift the tank - it glides easily enough with a bit of force (if I empty some water out of the tank).
Am I happy now?
I'll never tell.