The sunbrella fabric runs about 15.00 a yard,so figure out how much you would need,vs the price of a ready made one.I made a new canopy for my swing a few years ago,and that was easier and cheaper.I think it cost me around 50.00 with tax.Since then my cushion covers wore out,so for them and because mine sits out on the patio all year,i just use king size sheets and make new piloowcase type covers for those,then i can pull them off and wash as needed.
Kathi

I did one this summer I'll try and post a pic sometime.
I used heavy attractive drapery fabric from WM I got the cheapest I think it was about $4 a yard had to buy about 10 yards to get the "petals" I had an old pattern for an umbrella cover but just used it to get an idea of how to.
Actually it sits ovr the umberella I have.

It was a lot of work.
I ended up buying a whole new umbrella set when they went on sale from @$80 to $40.

I have saved 2 to do that but so far I haven't tackled it. I have seen some outdoor fabric ever so often on Walmart's $1 & $2 tables. May not be a pattern you like. I got 2 bolts recently to re-do the cushions on my daughter's chairs as their anniversary present. It's tan with a little white rectangle scattered across it. Kinda blah but won't clash with the other things.