I have a Nacra 5.2 and built my own set.
I used a couple of long pressure treated garden fence posts like 5x7 cm (approx 2x3"), just pine, nothing heavy.
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They are held at the right distance by shorter bits of the same wood. Then two pairs of largish wheel-barrow wheels ,Made in China (6! Euro's a piece) are fitted to a stainless tube which is recessed into the wooden spacers to either side and held there by an alu. strip and ss screws.

The redundancy means I can still move my boat, even with one or two flat tires.
This set up is not heavier than Cat trax as far as I can tell. I must admit thet Cat trax are still better on the beach though.

People have asked to borrow mine (more than once), so they must work! But I use them most on sand, mud and grass.

The caveat is, this is my second attempt, the first was waaay more expensive and both together set me back as much as a decent set of second hand cat trax would have.

If I had to start over I would fork out the cash for some second hand cat trax. If I was really strapped for cash, I'd build the above again.

Oh yeah, the boat hulls sit on the outside, the wheels are just inside the hulls. The bits that stick out (where the boat sits) are covered in el-cheapo doormat.