I made several sets of bucket/trash bin panniers for cleanup activities. When we do our annual watershed cleanup along the Potomac river we gather quite a bit of debris that is sharp and could puncture a bag or at least poke the goat ... so the hard sided trash bins work really well.
Here's a photo showing them in use ... hanging a little high in this pic, but should give you a sufficient idea:
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And here is a photo showing the strapping from the sides/back:
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I first bought the trash bins and then measured the circumference near the top and mid to bottom. Then stitched the "circle" straps (the straps in the pic with the white edges). I put these on the bin snugly and then took another piece of long strap to lay over and mark the intersecting points. This long strap attaches to the front top circle, crosses (and is sewn to) the bottom circle, goes under the bottom, up the back, crossing again the bottom and top circles and then left with enough length to attach a buckle. A small piece of strap is sewn diagonally along the side, from the front bottom circle to back top circle and again with some length left over for the other piece of the buckle.
Good luck!
Brian