A few thoughts on tapping mounts, using SAE hardware.

For every tap size, there is a specific number drill size that must be used.

For best results use a drill press and make sure the mount being drilled is flat when drilled.

Plastic is very easy to tap, either in nylon or glass filled nylon. No lubrication is required.

Aluminum takes a different technique. You run a risk of breaking a tap if you do not do the following:

Use a light oil or kerosene for lubrication

As you go forward, you stop and reverse direction every half turn forward. This breaks the metal that the tap is removing to form the thread in the mount. For the softer alloys, you may want to tap in 3-5 turns (in the 1/2 steps mentioned), then remove the tap completely and clean before resuming the tapping process. This is especially important with deeper holes, smaller taps, and softer alloys. I try to avoid hardware smaller than 6-32, even if I have to enlarge the mounting lugs holes slightly. It's just easier to tap 6-32 than 4-40 hardware.

With either glass filled plastic or metal, I don't find any type of nuts or locking washers to be necessary.