It would be possible using multiple SPDT switches, or a DPDT switch, but the aim was to go for simplicity, so I opted for an external module, which could be charged from mains using removable batteries (And much faster too)
Step 1: Rail Mount
First, two rectancles to undercut the gun's rails. They had to be seperate so they don't catch on the fake rail at the front of the gun.
Second, an elongated "U" shaped riser.
Last, the actual platform on which the electronics would sit.
The epoxy I used was pretty old, and kept coming apart, so I drilled some pilot holes at strategic intervals, and twisted/pushed some small brass screws through the acrylic, securing the layers in place
I sanded the sides down, and made a slight angle for the bottom layer to help it fit under the rails more easily, as well as the screws and acrylic's underside
Step 2: Electronics / Laser Mount
The battery holder, again wouldn't stay on with epoxy, so I partially melter a pair of screws into the dividing wall, through the acrylic.
For the laser, I made a small sandwich of acrylic at the front of the upper acrylic plate, and dremmeled a semicircular groove for it.
I also made a very basic on/off circuit with an SPDT switch, but only secured the laser module with blu-tak for the time being.
Step 3: Laser Calibration
Make a note of the average hit location, attach the laser array, and epoxy the laser into the slot with it pointing at the location on the wall. The priming mechanism on the Maverick is constructed fairly precisely, but will wobble slightly, causing the laser dot to vary within a ~15cm range, so try to centralise it before deciding what position to glue the laser in.
I held it in place with a rubber band and a paintbrush to lever the back up while the epoxy set.
Note: The red line in the second picture is only the paintbrush! You can see the dot on the wall!
Step 4: Results
The major downside is the weight of the whole module causes the priming mechanism to slide back and forth once it has been locked back (The spring to hold it forwards isn't very strong).
A far simpler method would be to drill a hole into the front of the gun, and make an internal laser mechanism as originally intended (Tutorials already exist for this), but to also attach a DC port for charging the batteries with a 7.5v mains adapter.
I can now add it to my ever growing collection of zombie defense weapons!