I didn't get very far with the adobe illustrator file so I re-drew it.
This is made for 6mm mdf for all you metric humans out there.
It evolved as it went (as these thing tend to do). So now ....
I advise you glue the base under the rear of the gun. you can either put a piece of paper under the cutout or put the removable plate under the nozzle.
look at the pictures for construction sequence & see how you go.