Start with a couple Mavericks,

Underneath the turret there are two nubs sticking out,

Saw/file/dremel them off so they are flush with the rest of the underside of the gun.

Next, put a strightedge up against the bottom of the gun and over the top of the handle. Mark this line.

Cut a couple centimeters below that line and you should end up with this,

File it down the rest of the way to ensure a good fit.Now it should look something like this,

That would be rather uncomfortable to hold, so now we need to cut off some of the handle
A comparison,

Now glue all together and glue a piece of plastic over that hole in the back of your handle.

Make sure everything is glued well, add duct tape around the handle for comfort. I use this as a side arm to my longshot so I put a strap on it, I highly recommend doing this because it allows you to operate both guns with one hand. Simply put the strap around your shoulder, and push down to cock, then shoot with one hand, as well.
A couple pics of the finished gun:


Comments/queastions?