I created my own custom size 3 sheet (6 page) wetnote from PVC sheets I bought on Amazon and printed formulas I wanted on 2 of the pages using waterproof paper from a Fedex printing store.
I formatted the formulas to optimize the page size in Word and then had the Fedex office print the info on waterproof paper for a total cost of $1.99. I glued the waterproof paper to the PVC sheet and then sealed the edges with clear nail polish as well as a coat on the formula print.
I've only used them in fresh water cave diving, but it holds up well and easily replaceable by gluing a new sheet on top or swapping out a new or different PVC page.
Here are the sheets (4 for $19) I purchased from Amazon and then cut down to the " x " size I wanted:
ABS0920 4pcs 2mm Thickness 200mm x 250mm White ABS Sheets 9.84'' x 7.87'' x 0.079''
The 2mm sheets were a good compromise to keep the thickness and weight down, as while they have some flex on their own, the pages can flip completely with the rings binding them which makes it very sturdy regardless of which page you are using.