I noticed you didn’t order a spray skirt with your deck, so I’m guessing you’re using a different skirt, which might be a different shape, so trying to force it on may cause problems. If it’s a skirt designed for use on a kayak with a fiberglass or stiff plastic coaming, that will have a much tighter bungee in it than the one I supply with the DIY skirts, and the DIY decks just aren’t designed to withstand that much compression.
I’ve tried several different types of plastic tubing inside the coaming, as well as aluminum tent poles, and the stuff I supply in the kits is the best compromise between stiffness and flexibility I’ve found (If anyone knows of something better, I’d love to hear about it though!). If you want to make it stiffer you could try using aluminum tent pole segments on the sides (order on Amazon and bend them to shape) and join them together with the plastic tubing – that might work, but if you’re using a kayak spray skirt, even that will probably not be stiff enough.
If the kink won’t go away and is causing a problem, you can cut the tube at the kink and join it back together with a bit of dowel or anything with an 8 mm outer diameter. I have some 8 mm tubing that will work for that if you can’t find something suitable.