Are we talking about a cover for when the bimini is folded against the backstay(s)? If so then it is very easy to make a pattern. I wish that I had some drawing software to illustrate. And I don't have pictures. But mine is usually folded and under cover. I made my bimini so making a cover was pretty simple.
Take an old bedsheet and on one side cut 12" or so slits for your backstays. The end of the slits will match where the top frame rests on the wires. The length of the slits should give you enough material to cover the folded up canvas plus three inches of overlap. Fold that over the top and down the back. Now cut the front side to match the length. Now you can see how to round the ends leaving enough material to sew a solid seam. You sew it inside out of course.
For the slits I used 2" velcro. It requires seams on the slits, reinforcement of the area where the wire hits, and an overlapping flap. I can provide details if necessary.
You need a seam along the length of the two bottoms. I roll it so that no raw edge is exposed. Or you can order welt from Sailrite like what is on the bimini. Then the seam around the corners and down the 12" ends. Then I sewed two inch wide velcro in four inch lengths along the bottom, maybe six places.
So you end up with what is a long pocket with two slits at the back. Easier to make than explain I can tell you.
P.S. just added a really lame sketch using M/S Paint.
