As I use quite a bit of cheapo ply, ~0.2" thick mainly from my local Lowes store, and most of it warps as soon as you look at it, I’ve needed a heavy duty hold down that I can laser cut.
I’ve tried the hold down pins, but found that they pull out with the strength of this ply, so I’ve designed my own.
Step forward the Warp Wedge.
Much stronger than it looks, due to being in tension, the lower hook locks onto the lower face of the honey comb, and the cut out wedge slides under the top hook giving a variable gap above the honey comb to pull down the warped ply.
Should hold down ~1/8" - ~1/4", but please feel free to modify to suit your needs.
This will give you 4 sets out of a piece of draftboard(recommended) 1.5" x 1.25".
Enjoy, but please let me know if you come up with improvements.
PS the inkscape file is in outline mode, my favourite for designing.