Just like some have said, add a plastic tube to the barrel of the slugga, and it's a shoota! And if you are wanting to keep it cheap, I use the ink tubes from ball point pens instead of buying styrene tubes. Works great! Just pull the pen end off and let the ink flow out...it can get messy, so have something to let it drain into. I did this on a Nob as shown below. Of course I also added a Burna to this one, but you can see how it works...
And if you want, you can add just a bit of plasticard to the back and give the shoota a stock for support.
Good luck!