Super simple, in hindsight!, to make. I didn't take nearly as many photos as I wanted.

I took three sheets of 2M x 70cm x 5cm insulation board and glued them together and made a paddle board.

Motivation:

SUP are expensive.
I live near a large lake

supplies/parts

three sheets of 2M x ~50cm x 5cm insulation board
- the boards are notched to fit snugly side to side not face to face!
4 aluminum T bars (1M)
strong glue (insoluble in water)
insulation pegs

tools (this is not complete but close)

clamps(the more the better)
wood saw
rasp
hammer
electric drill w/5mm bit for insulation pegs
utility knife
vacuum cleaner (the dust from sanding/cutting the insulation in pernicious)
some long straight sticks to clamp the whole thing
glue spreader
rubber/disposable gloves (the glue gets everywhere)

I got permission from the wife, of course, to make a mess of our tiny apartment first!

Step 1: Set Up Your Work Space

It's a step, yes, especially when you live in a small apartment and the kid and the wife are running in and out of the room.

I set up a piece of plywood (that was soon to be purposed as a chalkboard) on the kitchen table to add some length to my work space and to protect the table.

Take some time, do a little gedanken experiment to think through what you are about to do... and some of the issues you can imagine arising. This is especially true if you are planning on making an instructable! While working (at speed) the camera becomes an obstacle.

This instructable is a START and cannot cover all of YOUR variables.

Step 2: First Sheet Apply Glue Evenly on the Surface

I placed one of the insulation sheets on the table and began to apply a coat of glue. I should have used disposable gloves as my hands got gooky too soon.

I was told by the hardware supply guy that this glue would not deteriorate in water. I was told wrong (more on that later).

Make SURE the glue you use won't melt your insulation (too strong/acidic) and won't melt in contact with water (water soluble).

I had forgotten about my four aluminum T bars so the first sheet didn't get one.

Step 3: Second Sheet With Aluminum T Bar for Bracing in the Middle and Insulation Peg(s)

I put the second sheet of insulation on top of the first. The glue was not fast drying(I was told 30min. to set).

I drilled holes in what I considered stress points of where the board might bend under the weight of my wife and I(a well designed/purpoed SUP can easily hold two people). I squished some glue in the hole and pounded down the insulation nail. I did this eight times(between sheets 2 down to 1 and 3 down to 2).

I cut out narrow strips to get the T into the insulation as the board I was using was pretty strong and I couldn't just hammer the T in... (though, I did try~!)

After all was said and done, I don't think the T Bar was necessary but I did sandwich two Ts in between the 3 sheets of insulation.

Step 4: Put Glue on Top of Second Sheet

More glue (of the wrong kind).

Step 5: Third Sheet on the Second and Time to Clamp It

The need for even pressure between the insulation sheets is necessary. You don't want any air pockets to get in as that is a place where water can begin to pool and make your board very heavy.

The clamping/pressuring the SUP was by far the most difficult part and I would recommend you give more thought to the process than I did!

I do have cabinet clamps but certainly not enough deep throated clamps to get the pressure even across the entire SUP.

I ended up using a combination of dead water weights, clamps and pressure sticks to get the SUP glued up. It DID work but I was frantic for a bit trying to get weights/clamps/sticks correctly positioned.

Step 6: After 7 Days to Cure/ Dry

After the SUP had time to dry(one full week) I used a rip (wood) saw and a rasp to cut the prow.

You'll need a file rasp, a rip saw, a vacuum, a metal brush, a felt pen.

I should add that this insulation had super high propensity to stick to most everything, especially small particles. Keep the vacuum handy or you will have insulation minutiae pestering you for days!

As well, a good metal brush to clean the rasp as it gets gummed up with insulation particles is handy.

Step 7: The Paddle

I took a grid beam (Open source building system in wood and metal for creating a post carbon future, from their site gridbeam.com) and attached a piece of ply (cut more or less into a paddle blade shape) with some old bolts I had laying around.

It was fairly heavy as the beam is 1.5M x 50mm x 50mm but it worked!

Step 8: Testing

Yes, it worked. I climbed up and paddled across the water without issue, initially...

No, I was not happy with it.

I didn't carve a good V shape on the bottom so the SUP listed pretty hard. I didn't add a fin. As well, it was too narrow for me to stand with legs perindicular to prow.

The glue that I was told would NOT melt in water began melting almost immediately even after a week of curing. The locals were not impressed with the white trail I left in the water...

I made my escape quickly when I realized the glue holding my little craft together was in fact melting.

So, the paddle board ended up getting cut lengthwise to support our treadmill walking desk I am using right(write) now. (The insulation dampens the motor noise/tread noise for the (unhappy) neighbors below.)

Next season, I'll use a clear water proof silicon glue to bind the sheets together and will keep the rest of the steps. I don't know how I will get an even pressure gradient across all of the sheets just yet, but at ~70+ USD I can afford to experiment again.

Please let me know if Instructable was helpful/interesting and if you have any tips on how to build a cheaper/easier SUP please let me know!