What about buying a jacket that is comfortable and then buying some reflective tape to stick on in a V down the back (very effective from a drivers point of view) or some reflective material and tacking in place...?
I have done something like this for one of my sons.

I have also seen reflective aprons worn in reverse like a bum-bag (I think Americans say fanny pack???) - I think these can be bought, but it would be relatively easy to make one

As a driver, I find these "aprons" very effective as the lower back and backside of the cyclist are extremely visible from behind (the one that is best is red and silver vertical stripes)