I have two covers by Max Dahl of Minnesota. Google Dahl's Canvas Covers.
Custom made (I supply the boat's dimensions) of marine grade UV resistant canvas w. belly straps and cinches. Excellent work for $89.95 plus $10 shipping. One of mine is 4 seasons old and looks just like the new one I got last year. I did launder the first one and it came out great.
Canvas breathes, plastic tarp does not. This won't matter for the plastic boats, but it will if there is any cloth in your seating.
Danuu is a good company, I chatted w. their VP at a paddle show, but their standard covers are too wide for my boats.