Using a subwing is a lot of fun and provides a great way to explore under water, and see sea life in a different way. Building a subwing is also lots of fun and easy. I've already built several of them and improved the way to build in order to make them durable.

Check one of my earlier prototypes in action https://youtu.be/ChKxhbyTYcE

Check the commercial product in action https://www.youtube.com/watch?v=KLaBXMvDVU8&t=129s

Step 1: Supplies and Tools

What you'll need (mm = millimetres)

Plywood 15mm (thick) 1200mmx600mm (will enable you to build 2 subwings. You can also buy 600x600)
Stainless steel screws 6xL25mm and nuts (image incorrectly shows 5xL25mm) (item #2 in image)
2x Stainless steel flat-surface bracket 15mm width and 100mm or 120mm height (to be used on the side to fix the axis)(item #6)
4x Stainless steel flat-surface bracket 40mm x 100mm (to be used to "sandwich" the axis) (item #5)
1 x Stainless steel threaded rod for the axis 200mm long and 8mm diameter (item #7)
2 x 8mm stainless steel hex nuts (item #4)
Stainless steel wood screws to fix the brackets to the side (item #3)
10m of rope (item #1 in image)
Rubber for hand grabs
Rubber tube for back rope
Waterproofing Wood Sealer or Varnish to paint

Tools:

Screw driver.
Drill and 8mm and 4mm wood drill bits and 8mm metal drill bits. (items #9,10)
22mm diameter round drill bit (item #8)
Wood Sander
Wood saw
Metal saw to cut the stainless steel threaded rod

Portuguese (mm=milimetros):

Contraplacado 15MM 1200x600 (dá para fazer 2 subwings).
Parafusos inox 6xL25mm.
2 x Placa Junção Inox 120mm x 15mm.
4 x Placa Junção Inox 120mm x ~40mm.
Varão Roscado Inox 8MM diametro e 200mm comprimento.
2 x porcas inox 8mm.
1 destorcedor Inox.
Cabo e um tubo para ser mais facil agarrar o cabo.
Espuma para o sitio de agarrar as mãos.
Resina Epoxy

Ferramentas:

"Tico tico" ou serra madeira
Berbequim
Brocas berbequim
- Madeira 8MM e 3MM
- Metal 8MM
- Broca madeira redonda 10mm
Lixa e Lixadora
Chave parafusos
Serra de metal para cortar o varão roscado (eixo)

Step 2: Cut the Plywood

If you bought the 1200x600 wood, it will enable you to build 2 subwings.

With a pencil, divide the rectangle in halves, horizontal and vertical. Then draw the subwing on one side, matching the design you wish from an online picture or from this picture.

Step 3: Sanding

After each side of the wing is cut, you'll need to smooth the edges. Think that the water will flow from front to back, and smooth the edges so it's "wing like".

Do not sand the "inner" part where the axis will connect both wings.

Step 4: Side Bracket - Drill a Hole for Axis

The stainless steel side bracket will help keep the axis in its place. You'll need to drill a hole for the axis in the flat-surface bracket protection. The hole should be the same measure as the axis. 8MM if you follow this guidance. After drilling the hole on the flat bracket, use it to mark the wing where the hole is. The Axis should work around 6 cm from the top edge of the subwing.

Step 5: Drill the Axis Hole

After using the stainless steel flat bracket to mark the whole, you'll need to drill the hole for the axis on the wing. The whole should have the same diameter as the axis, and you need to really center it and make it perpendicular to the wing. You could use another object to help assure the drill is perpendicular to the wing. You also need to be parallel to the wing, if not the bit will come out on the side of the subwing.

Drill to a depth of a[[approximately 3-4cm (the length of a normal drill bit)

Step 6: Drill a Round Hole for Nut

Measure where the axis hole ends, and cut a round hole so you can screw the nut on the axis.

Step 7: Insert the Axis and Side Bracket

Measure how much the axis enters the wing on one side. Leave around 2 cm for the junction of the 2 wings. Add the same measure for the other side and cut the axis.

Then insert the axis on one side. Insert the side flat bracket and screw it to the side of the subwing. Do the same thing for the other wing. Remember to insert the bracket on the axis before you insert the axis on the other subwing,

Step 8: Add the Brackets to Both Sides of Each Wing

In order to keep the axis inside the subwiing, use 2 flat brackets on each wing, one on each side of the wing, and screw them together using the 6xL25mm stainless steel screws

Step 9: Add the Front Rope

Draw two holes, aligned with the axis, and 20cm apart. Pass the rope through the hole, and tie a figure 8 knot so the rope will not pass the hole. The rope should go 2m to the front and back into the other hole on the same wing.