I've found an inexpensive (how's free) material that makes a great fillet board. I stumbled across it two years ago when a new home was being built next door to me. I'd just got back from fishing with a few fish to clean and I really didn't have a good fillet board. Well you know, a house under construction has all kind of possiblities so I wondered over and found a piece (14" x 30") of left over exterior sheathing. You know the kind. Press wood chip with exterior glue to be somewhat water resistant. So I took it home.
Well, two years later I'm still using that board and it has gotten better! With the water exposure to it, the wood grain has come up and talk about a non-slip fillet board. You don't need to worry about fish slime or blood making a slippery surface. It cleans up in the backyard with your high pressure nozzle on your hose and you can make it any size you want.
I'll never buy a fillet board again. I'll just visit a new fab house construction and beg a piece of scrap of exterior particle board from the construction crew. If I don't lose the current one that may be in 15 yrs!