You should have 2 pieces...the tray itself and a bottom piece. The way I mounted it was by taking the bottom piece, placing it on the chassis where you like it, clamp it to the chassis to hold it in place. Then drill 4 holes thru the plastic and the chassis. These are your mounting holes. You will need to counter sink the holes on the bottom of the chassis. Next, I took the tray itself and screwed it to the plastic bottom piece. You will need a piece of foam or such to put in the bottom of the tray to keep the batts. from hitting on the screw heads.
Some of the trays I have gotten from Mike are just one piece so just line up where you want the tray, drill, countersink the chassis holes and you are done! The velcro straps fit nicely between the tray and the chassis or between the bottom piece due to the nice groves Mike provides us with.
Good luck...and really it is quite easy, just take your time
1. MBX-6 T8 1900KV, RX8 ON 4S
2. MBX-5T 1520, MMM ON 5S
3. MBX-5 ONROAD CONVERSION 1515, MMM ON 5S
4. MRX-3 ON ROAD CONVERSION 1512, MMM ON 6S
5. TEN T 2650 T8, MMP ON 3S