Whether you're staying with the 4" or going to a 4x10 or whatever, I'd do a patch panel underneath with the appropriate holes. Rather than sheet metal (prone to rattling, tinny, bad for sound IMO) I'd use a sheet of plastic. Due to my various projects I'm on a first-name basis with the local plastic place, but you oughta have a cut-to-size place local to you. (If not, https://www.tapplastics.com/
ships! ) I'd think an appropriately sized piece of, say, 3/16" or 1/4" ABS or styrene would do nicely. I'd prefer that over acyrlic as that stuff is harder to work with and more brittle. Measure and mark carefully and then go to town with your Dremel.
Hell, in a pinch you could do like 1/4" hardwood plywood from the Home Despot cut up with a jigsaw. I'd also just glue the boogers in, Liquid Nails or Gorilla Glue. I know it sounds totally hack, but again, you want something that won't come loose and rattle the way a screw or bolt can. Especially if you're going to 4x10's, there should be enough room to clamp it well.
As shown with the metal one, I'd install it on the bottom side as opposed to the top. It is painful to get to, especially the driver's side, but the grilles are a tight fit enough up top without moving the speakers up any farther.
This is the joys of buying old cars; you often wish you had a "previous owner seeking missile."
[This message has been edited by dremu (edited 01-27-2021).]