Front-mounting connectors is easier than rear-mounting them: even if you don't make a perfectly polished hole the connector frame will hide the imperfections. You could do as follows:
take a frame for PC connectors like
this and use it as a template to mark the hole outline with a stanley knife (better than a pen). Next take a dremel and drill 4 holes near the corners of the outline and use a jigsaw like
this to cut the plastic from hole to hole going as close to the edges of the outline as possible. At this point it's all a matter of smoothing the hole with sand paper until the connector fits the hole. Put tape all around the hole to prevent scratches.
As a totally different alternative for your project, you could remove the small PCB inside the DC stick and make an external adapter out of it and replace the original cord with a PC one that will plug onto the supergun or the adapter. I think this is a better solution because you don't have to ruin the stick and you'll have only one cable coming out of the stick.