You can make a pretty good one out of a standard 11" black Dura-Block.
I used a radial arm saw (table saw would work just as well) and cut some slits on the back (handle) side of the board. I put 2 - 3 about 3/4" deep long ways and one about every 1 1/2" - 2" cross ways. You can vary the flex by the number and depth of cuts. Start shallow and fewer cuts and go deeper and more cuts to get it more flexible.
Keep your brain in gear when you're making the cuts. I screwed up my first one because I wasn't paying attention and made the slits on the sandpaper side of the board.
I think it works as well as the high $ one from Eastwood.
You can build a board with 1/4-3/8 plywood. Don't foget to round the edges and make sure you have foam to soften the print and also the cover needs to be able to accept the PSA paper. And watch your fingers when setting the "Flex". I've built boards over 3 ft for use on boats. I wish I knew then what I know now. Check out the web site.
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