It's 1/8 inch Masonite (same material the front kick panels were made of)...
1/4 is Too Thick... just saying... 1/8 Masonite is Very inexpensive at Home Depot .. etc.
This is the panel directly over the spare tire... is this the one you mean??
Note: this is out of my 65 Barracuda Formula S that came with 14 inch wheels...
Or is the compartment fold down panel you are wanting ?
This would be 1/2 inch thick plywood...