I had not seen the Pakrat cover. Last year I made my own after looking for one and not finding it.
I used four yards of black duck cloth. This is the material that insulated coveralls are made from. It is breathable and washable. I did not want something that would retain water. Waterproof material would trap moisture and not wash out nearly as well. Not to mention the condensation that may form inside if moist items were stored there. I want stuff to dry out if possible. I had a choice of black, brown or off white. Black won. I have kind of a black thing going with my truck. Sure it might be hot, but it will hide stains better than the lighter colors.
I bought a heavy parka zipper 26" long, heavy thread and some brass grommets. I already had a good stock of 3/8" and 1/4" shock cord, so I was all set there.
It took about four hours to put it all together, including two trips to town for materials. 110 yards of thread was half what I needed

All seams are at least double stitched. And it is double layered everywhere.
My spare is a 33x9.50-15. The cover fits snugly.
The grommets in the bottom are to aid in draining so that I can just hose it out. I also "Vee'd" the bottom back side so that road dirt could be washed down and out.
That is a 3L can stuffed in there.
All together I had about $50 in materials. It has been very handy.