Hope you feel better soon Chris. Now that I've seen the cannon you mean, how about trying this, should work in 1/48th scale, given there's enough thickness of plastic. Drill-out the cannon troughs, holding the drill parrallel to the fuselage to create the trough. If it 'falls through', leaving too deep a hole, back-fill the hole, from the inside, using Milliput or similar. When the putty is almost dry, press-in the shape of the trough and leave to set. When hard, carefully ream out the trough, using the drill bit, but not the drill. For the cannon barell, only the last part to the muzzle of which is shown, if you can't find a suitable barell, or tube, use plastic rod or stretched sprue and, if possible in this scale, drill-out the end. If not, paint the end in a dark silver/steel colour, with a central balck dot. Looks pretty convincing as an open muzzle. I've used the above approach before, and it worked well. Just a thought!