There are three types of L-brackets that look similar. We need to take the two types shown and described below. To these need to be added M5 x 8mm bolts and T-nuts.

M5 x 8mm Button

M5 T-nut

Do not tighten down yet, just fasten on the T-nuts by a few turns.

The first type has one 6mm hole in the corner as shown. (x4)

The other type only has 5mm holes around the edges. (x12)

The third - which is not used yet - has a threaded M4 hole on its diagonal edge. (x4)