My 96 xlt has a heavy OEM bracket behind/above each fog light.
The aftermarket fogs I'm using have an adjustable angle bolt thingy (technical terminology) that I modified slightly to use the OEM brackets.
I had to put in a longer bolt, add a few extra nuts & washers and file the factory hole out (to a more oblong shape) to get my fogs to sit in the OEM location properly.
I located the bolt flanges at the bottom, with the bolt facing backwards through the OEM bracket. The OEM fogs mount the same way, but aftermarket fogs don't have the little spacer thing for the bolt, so you might need to add a few extra nuts & washers to get the lights to set flush (or slightly recessed) in the valance.
If you don't have the OEM brackets already, you could easily bend some 1/8" thick metal to make your own brackets to hang down from the bumper. Probably a ] or Z shaped bracket from the bumper should work. You could probably turn the mounting bolt upwards (closer to the bumper) so your brackets would be shorter.
I believe the 89-93 era mustangs use the same OEM fogs & the brackets are similar.... if you have a junkyard around close, you might be able to scavenge a set of OEM brackets.