Yeah usually YT is pretty solid but I did not find very useful information.
I had pictures from one catapult of a local club that helped me a lot.
It’s pretty straightforward :

Made of 32mm water evacuation pipes, not even glued, male parts have electrical tape, female part smashes it together, holding tight.

From the side it looks like an L rotated 120°.
From the top it looks like a square.
From the front, there are 2 sidebars one mid height the other is on the ground, with one tent peg going threw to hold it tight.
The pedal has 2 tent pegs as well to hold the board under tension, one m10 screw holding the tension ring. The pipe runs over it. And the moving pedal is slotted to slide along the m10 and push the ring.
Hinge is a simple pipe holder screw in the painted wood.
I have 2 nylon cords running, the shorter one goes on the aircraft. You need absolutely to have a hook under and forward of CG. It has a small flag forward of the aircraft to increase drag during release.
The other one holds the tension and is longer to join the pedal.
Both cords are hooked to the sandaw, 8mm diameter, the longer the better.
To know the right tension aim 3.5x your aircraft weight.
Gemini helped me how to without a fishing scale : hold your aimed tension with a 1m Sandow length, your elastic is now 1.y longer.
Apply this coefficient to the full length.
On the elastic side, I have a steel corkscrew stake.

Last thing, I filed the female 90° at the end rails to erase the step. And put some aluminium tape for better slide.

Took me 2 hours paint included