I didn't make photos of the steps, but I will highlight on the assembled kit each of the steps you have to follow.
Step 1: What you will need
Something to cut plastic: I used my swiss army knife.
File to soften the edges.
Drill to make holes in plastic. Wood drill will do, it has a pointer to help keep in position.
Electrical fast ties.
Straps, and buckle from an old backpack.
Step 2: Cut, drill, file
So the aim is in the final product just to have as small flat part as can.
Also cut two additional parts from the plastic plate with the same widht, and height as the mount: that will fix the strap in place, giving more stability for the mount.
File a bit the edges where you cut, not to have sharp.
Make 4 holes in the mount, and also into the two additional parts: make sure to align the two parts to the mount, and mark the holes before drilling.