Our Madsen rain cover frames are made from electrical conduit, specifically 1/2" EMT. It's light, cheap, durable, and very easy to work with. You'll want to use a pipe bender like this. You may be able to rent one from a home store or borrow one from a neighbor. It's a basic lever hand tool that is very easy to use. Measure twice, bend once. And make sure you are bending the conduit in the corrected direction!
The frame has only four pieces: two 10' lengths of 1/2" EMT (conduit) and two 1/2" couplers. Total cost is maybe $5. Ideally, the left and right sides of the frame are mirrors of one another and the couplers line up in the middle of the bucket. The structure sits on the edges of the bench seats; both benches must be used for the cover to stand on its own.
The frame can be covered with anything. I recently discovered that a burlap coffee sack from my local roaster is the perfect size and shape when you rip the side seams open. I added a piece of coroplast to the roof of this one to help dome the roof and take out the slack in the fabric.
To keep the whole thing from tipping out of the bucket, you need some sort of tensioner under the outer lip of the bucket to attach to the cover fabric. For our sewn covers, we used Sunbrella fabric and hemmed in a slot at the front and back to loop through a 1" web belt with some buckles.
In a pinch, the burlap coffee cover was attached with a length of rope and some zip-ties. Total cost of the burlap cover and frame: less than $10 and 2 hours of playing in the garage. This makes for a decent sun shade or windbreak, but wouldn't last long in heavy rain.