My list includes just what I used to make mine, but I offer suggestions for variations based on your preferences. Everything you need can be found at your local Home Depot (Only because I work at one)
1) 1 in. x 6 in. x 8 ft. (cut to 4.5 ft. or preferred size) PINE COMMON BOARD ~$8
The size can vary based on your preference. I had an 8 ft. board cut to 4.5 ft. to mimic the width of my bed I was used to chopping out on, and allows ample room for comfortable octaves in both hands for some versatile practice. You may choose to go wider to account for the accidental manual; or longer to accommodate the horizontal reach of the naturals; or maybe thicker, I don't believe it will differ in feel, though it will in sound. KEEP IN MIND: Whatever specs you choose for your board, you'll have to make sure you have enough rubber (see 2) to cover the surface.
2) HUSKY 60 in. TRUCK BOX LINER (SKU 644177) <- give an associate this number if difficulty finding ~$15
This was a tough call, and still might not be the best option out there for a reasonable price; it was just what I could get my hands on as easily as I cared for. Comes in a roll 17x60 inches, super easy to cut to the size of your board with scissors or box cutter. NOTE: This liner is fairly thin; one layer ensures that it'll still sound like you're playing on a wooden plank, but provides protection between mallet yarn and wood. I'm still experimenting with adding a layer, and still searching for another form of rubber (similar to drum pads) for a decent cost. If you happen to find a better resource, let us know in the comments!
3) 3/4" FLAT HEAD WOOD SCREWS
Any will do, really. 3/4 in. will easily secure the liner to the wood without poking through one side. I just got one pack of 12, enough for 6 on each side including four corners. Be sure to get the appropriate screw length for the thickness of your board and whatever rubber surface you decide to use.
OPTIONAL: You may find an adhesive to be easier for securing the rubber to your board. If so, leave it in the comments for everyone!
Sanding block/sandpaper ~P150
Once you have your board cut to preferred size, you'll want to give it a good sanding to prevent splinters snagging on any carpet or bedding. P150 or any medium-fine sandpaper should be fine. Sand the faces, edges, and corners to your satisfaction. Wipe off remaining sawdust with a rag.
Be as precise as you like, but remember: measure twice, cut once. Common board may not be as square (straight edges and 90 degree corners) as possible, so make sure all wooden surface will be covered by rubber to avoid yarn-wood contact while playing. Allow some excess on one side when measuring your cut. When screwing down one side, you'll be able to trim to open side for a more precise fit. TIP: Lay the rubber out flat, line up one side of your board with the edge of the rubber and trace and/or score the opposite side on the rubber.
Best done with power drill with screwdriver bit. Start with one side; line it up and screw down, brushing out rolls or folds as you go, and trim the opposite side to fit if need be before screwing it down. AVOID over-screwing, this may lead to bunching in the rubber that results in uneven bumps. TIP: score an X in the rubber at each screw point with a box cutter/scissors in effort to avoid bunching. Be as liberal or conservative with screws as you like. (I used 6 on each side; 4 corners, 2 mid, 2 splitting)
All done! Mine ain't too pretty, but yours can be. Find the prettiest wood you want! Or paint something meaningful on one side. TBH the final product of these steps is, as I mentioned, still fairly loud and inconvenient in size (relative). This version is more or less the bare minimum of what you need. From here, feel free to adjust, add, and enhance your pad. If you use the rubber liner, you might have enough left over to add a layer to the same or opposite side. Totally up to you! Happy practicing!