I finally installed the rubber feet on my 1×12 diy, home made speaker cabinet. I was thinking about putting metal corners on the cabinet, which would have made placing the feet very easy, but then I decided to not do the metal corners, so I had to come up with an alternative method to place the feet in the same spot on each corner.
I took a template my father-in-law had made for installing cabinet handles to try to keep the feet in the same place for each corner. It is just cardboard stapled to two pieces of scrap wood that make a corner. I measured and remeasured many times to make sure it was a right angle.
Then I placed it on each corner and, using a screw driver through the cardboard template, made an indention on the bottom of the amp. I repeated this three more times to get the exact positioning of each foot.
Then I predrilled a small hole and screwed in each foot.
Perfect (or close enough for me)!