Rail-to-rail balance boards have a long roller that goes down the length of the board, which more accurately simulates a surfboard trimming across a wave face. I made one when, after a decade+ of surfing I decided it was time to learn to longboard—this balance board definitely improved my cross-stepping. I've also used it to work on surfing goofy-stance. I don't stand-up paddleboard, but I imagine it would also help SUP too.

Similar boards are sold by the brands Goofboard and Revolution Swell, but you can make your own for a few bucks.

Step 1: Materials

3 feet of 3" PVC pipe
Plywood at least 45x15", 5/8" or 3/4" thickness (Consider a 4' x 2' handy panel?)
12" of small wood blocking? (optional)

Roller: You want the roller to be between 3-4" in diameter. PVC is measured by the inner diameter, so 3" PVC has approximately a 3.5" outer diameter. That works! Much taller than about 4" is scary. Much smaller than 3" and you risk hitting the edges of the board on the floor while you're riding. Another roller option is, fiberglass cloth comes on these thick-walled cardboard tubes about 3" in diameter...that will work, although I don't think it will last as long as PVC.

Plywood: Mine is 5/8" thick, which works well for me and every other adult who has tried it. In a pinch, you may get by with 1/2", because the roller runs lengthwise and the board is 15" wide, there's not a long span for the plywood to flex over. Otherwise, upgrade to 3/4" and you will definitely be fine.

Update: user Brunswick27 made one and said, "I used 12mm [~1/2"] plywood... It is OK with the roller lengthways, but I would definitely use thicker wood if making again." You can see their full comment below with more helpful tips, thank you!

Blocking: I used poplar scraps. You could use offcuts from your plywood, or a short length of 1x2.

Step 2: Cut Out the Board

I've drawn five surf-inspired templates to help you shape the board. Pick your favorite or create your own. The dimensions of all the templates are 45"x15"

I've created them as "spin templates" which combine the nose and tail outlines into one template that's 1/4 the size of your finished board, which saves paper and ensures everything is symmetrical. I'll describe the process of using them below, but the first step is to print them out. Download the PDF and print it 100% on three sheets of paper, use the registration dots to align and tape them together. A large window or sliding glass door can help you align the pieces. Then cut along the red and blue lines.

Once your template is cut, follow the steps below, which I illustrated both on the spin templates themselves and above.

All the templates fit within a 45"x15" perimeter. Draw a rectangle of those dimensions on your plywood, and also draw a longitudinal and vertical center line. The longitudinal centerline is referred to as "CL" in the drawings, and is also called the stringer line in surfing lingo
The blue line on the spin template represents the outline from the midpoint to the nose, and the red is the outline from the midpoint to the tail. Align the nose stringer point with the CL on one side of your drawing, and the nose widepoint with the outer perimeter. For all shapes without swallowtails, this will hit the vertical center line, for the swallow-tailed Fish and MPH shapes it will hit the perimeter slightly forward of the vertical center line—don't sweat it. Trace the nose outline.
Flip the template over the CL, aligning the stringer point on the nose, and the widepoint with the outer perimeter on the other side. Trace that. You're halfway done.
Slide the template diagonally across the board, aligning the tail widepoint with the outer perimeter and the tail stringer point on the CL. (If making a swallow-tailed board, be careful not to go beyond the 45" length, the swallowtail means your "tail at stringer" point will hit the stringer forward of the finished length of the outline.) Trace.
Flip over the CL, align, trace the final shape. Done!

Cut the outline with a jigsaw, stay a bit outside the line, then sand down to the line. When sanding, consider using a "fairing board" which is a piece of ~100 grit sandpaper glued to a thin, flexible piece of plywood, approximately 4" x 12" x 1/8". Using a fairing board will correct any wobbles in your outline and give you nice fair curves.

Step 3: Stop Blocks

You'll want to add some stopper blocks to keep the board from rolling completely off the roller. On my board I tried to get fancy and run these the length of the edges, which proved to be a waste of time and made the board harder to ride.* Instead, just add four small blocks to the bottom. Blocks should be approximately 3" long and 3/4" thick, maybe 3/4" wide. Emphasis on approximately. You could use scrap 1x2s, or offcuts from your plywood outline. Refer to the diagram and use your two center lines to mark out a rectangle about 11" wide, and 12" from the nose and tail. Place blocks outside this rectangle, glue and screw them on. Depending on the size of your blocks, you may need to cut them to prevent them from overlapping the outline.

* The stops on my board are right on the outer edge of the board, which I thought would look nice. It does, but it makes it hard to recover when the roller hits them, because by the time the roller hits them, there's very little board width to lever against. Put your blocks about 5.5 inches or so from the CL and that will make the board easier to recover. Have a look at the bottom of a Goofboard or Revolution Swell and you'll see their stopper blocks are inset a similar amount.

Step 4: Roller

Your roller should be 3-4" in diameter, at least 3' long. You want the roller to be shorter than the length of the board. If it's longer than the board, when you're hanging toes over the nose, you can run them over! I know first hand!

3" PVC pipe (3.5" outer diameter) works great. If you go much bigger it's scary; much smaller, then the board can hit the ground while you're riding. I had access to a wood lathe so I turned decorative caps to dress up the ends. Cute!

Commercially produced boards have some traction on the bottom of the board. I didn't add anything to the bottom of mine and I've never had the roller slip, but if you want to you could perhaps attach some strips of cork or rubber sheet with contact adhesive running side-to-sidebetween the stop blocks. My suggestion would be to try it without, and add them later if necessary. You could even try strips of cloth athletic tape, aka hockey tape, if you have some?

Step 5: Ride

To get started, it helps to have a chair nearby to hold onto. Don't put your feet straight down the middle like riding a skateboard or surfboard, that'll make it impossible to recover if you roll too far to one side. Instead put your back foot so your toe is touching the edge, and your front foot so your heel is on the edge.

To get on, I like to put my back foot on the board, toe near the edge, and position the roller under the arch of my back foot. I apply weight to my back foot until the board is about level. As I step on with my front foot, I'll angle the board so the roller is slowly working its way towards the middle as my back foot steps on. Hold onto a chair back until you get the hang of this.

Once you get the hang of balancing, try cross stepping. It feels very similar to a surfboard, but it's harder on this, so if you can get it wired on this thing, you'll be confident on your longboard. I've had no luck with spinners yet but have figured out switching from regular to goofy stance and back (that's half a spinner, right?).