Actually, the one brand of trunk bags I have found that don't sag are the Topeak ones. I think it has to do with the very solid plastic extrusion on the bottom that slides on the to alloy extrusion on their racks. The outer casing of the trunk is attached firmly to that, and the sides have closed-cell foam as additional support in the sides.

A friend had a trunk bag similar to the ones discussed here, a Nashbar one. The Corflute/Coroplast solution worked well, but!... in the bottom. I actually used the side of a plastic 3-litre milk jug. It was a tight fit in double layer bottom (I had to slit a hole in the front to slide it in). I didn't fit any side support. The bottom piece of plastic has worked very well. I think that if you just fit a piece of Corflute/Coroplast in the bottom without slipping into the fabric envelope as a tight fit, you need to used short bolts, washers and screws at either end and at the middle to attach the lot together; this stabilises the fabric the same as the envelope or pocket would.

As far as I can tell, you can cinch down the bags tight on the rack, but if there is not a solid base, the fabric will always allow the bag to sway around.

My friend has reported no problems whatsoever since I did the job.