I put this in my review but might as well upload it here. It might even work foor the original Flash's rifle though I have tested it. Anyway, here goes!
You'll need a vintage type hose (think Airtight or Tele-Viper). Cut the end of the cord just a bit and glue it in the BP so you have a little bit of cord hanging out. Then do the same with the rifle end. Leave just enough so you have some "cord" to plug into the tube. Now you have a perfect Flash pack and rifle.
End result:
I also cut a small sliver off the handle so it fit in his hand better. Can't be too careful when you're carrying a laser rifle!