Back in '88 a school friend & I built one from scratch in his parents garage

. At 17 we had no idea about working with foam or fibreglass but decided to give it a go anyway. We started like you but the 'design' we created was as simple as laying my board (John Hall 295) and my mates board (Bombora Astro Toy) on top of each other and sliding them backwards and forwards until the rails lined up!! The astro toy was at the back - it ending up being over 13foot long and about 700mm wide.
Next step was to glue 2 blanks together which turned out to be little harder than we thought....we had a nice join on the deck but the bottom had a 50mm gap (we cut them at a 45deg angle). No problem - fixed with builders bog and duct tape!!
Stringers - this we outsourced to the local surfboard blank company who I am sure thought we were not all there when we asked for 2 x 50mm stringers cut into this monstrosity
Mast tracks and foot straps - again a lot of thought went into this improtnat step - we simply put them where they were in the boards we used as a template.
The all important bottom shape - a double concave with steps on the rails - and when i say steps i mean about a 20mm step!!!!
Fin - same deal for the fin box as the footstraps but we splashed out on the latest fin design at the time and bought a Multifin slot fin.
Cant recall much about the glassing process only that we raised the floor of the garage by about a foot with all the excess

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At this point we didn't even know if the thing would float let alone be able to be sailed, bbut we decided to name it any.....the "MAJK BAJK (BAJK being a combination of our initials)
First test - this launch was done with much fanfare with family and fiends all gathered at Narrabeen Lake to see if what we made (whilst we were supposed to be studying for the HSC) would actually work.
Well....it did float and went like a dream

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We set it up with 2 wave sails and had an absolute ball.
Gybing - all i can say is you need to know your partners skill set very well and give yourself a lot of room (back sailor duck gybed) but it did do carve gybes after a while of dialing it in.
Not content with keeping it on flatwater, we took it for a spin at long reef in gentle 1-2foot to see what we could do....well...the first jump was enough to scare the s*&% out of me. As I was up the front I was 6 foot in the air before the back half had even left the top of the wave and the flex was enough for us to keep it as a flat water board only.
I still have it under the house nearly 30 year later and yes it still does come out occasional in a strong westerly in the Lake and even though my design partner has stopped sailing there are a few willing guys to take it. Here is a photo of about 5 years ago of me and a well regarded wavesailor giving it a blast.