They are Ironduck spineboardshttps://ironduck.com/product-category/p ... ineboards/
I put pieces of spaflex tubing that I split with a jigsaw over the akas. The spaflex has a somewhat tacky surface. The spineboards are just lashed onto the spaflex with 2 Ft straps from Austin Kayak. I seem to do it a little differently each time but the handholds on the spineboards make a nice opening to run the straps around in figure eights or whatever works.
From a PM in case others want this info:
Iron duck Spineboards I have:
35755-RP ULTRAVUE 18" -RED W/PINS 2.00000 2.00000 204.60000 409.20
I chose the wider 18" ones but others on the forum seem to prefer narrower Hakas and would probably go with the 16" ones. I don't regret getting the larger ones which are also significantly heavier but weight isn't a concern for me.
I also think the Pins are very useful, you can clip a conventional kayak seat right into them just need to add a strap at the crease between the seat and back so they don't slide. To use bungees on the pins you need ones with smaller radius hooks to fit in the pin recesses.
Note that with a 200# ++ person sitting in the middle in the hot sun these spineboards will sag a little but return to flat if you just turn them over. I have considered adding a beam across the bottom but my weight doesn't cause them to sag and I tend to sit close to an aka where they have plenty of support.