Thin plastic packing
Have a look on ebay for some tufnol. It does come in 1mm and is pretty friction free
How much are you doing. Full case both sides - formica was used in the past but it is pretty expensive. I spaced out the board with acetal strip (you can get various thicknesses from ebay. Epoxy glued and small countersunk screws to hold it. Board doesn't flop about now and a big help in pointing. I have given a link but I'm sure there are other thicknesses
if you contact your local laminate distributor and ask them for cheapest 1mm sheet they've got or ask at a joinery workshop for some offcuts.
just a thought would the 1mm lexan or 2mm acrylic clear sheeting I sell for transom flaps do the job?
Cheapest? A couple of old CDs glued to the sides of the board. Being round, they dont get in the way when lifting / lowering the board.
And according to the marketing when they were launched, they will still play when you have finished with them!