Struts, like used in Guitar top bracing could be used OR

Wellllll- sticking my neck out here.
If you can enclose it in a frame of some kind during it's application,
that might help OR !!!

Try this: Weigh your board
Dampen the board-- both sides
Find a REALLY flat surface-- maybe your TS
Lay down newsprint ( NOT NEWSPAPER- newsprint is the paper they make
newspaper from - get some at the local crafts supply - paper towels
work, but use more-- an extra layer)- maybe 5 layers (plastic on the TS
or it will rust)
Lay down your board
lay down More newsprint
add a REALLY flat piece of Steel same size or larger than board--
Aluminum will work, MDF??? maybe if it is about 1 1/2 " thick (two
layers)
If you use MDF or plywood put plastic between the newsprint or wood on
top will warp also
Clamp like HELL-- Really full 5 gal paint cans come to mind.-- use two
or three

After 24 hrs change news print for dry stuff-
Keep this up for at LEAST three days ( make sure your 1/8" piece of
wood is dry) Keep weighing your board- when it weighs the same as when
you started, it is sstable -- USE & finish immediately (if it's flat
enough for your purpose

If this is TMT, buy a flat board