Update: Demonstrations are back, although not in the same form. I've found a (not really) new GIF hosting service called Gifyu and set up an account for it after two successful test uploads without one. From this post onwards, new projects with electronic/mechanical features - but not the previous ones - will be demonstrated with linked or embedded GIFs. If I feel they might cause loading issues or slowdowns, I'll only leave links to them like I did when Gfycat existed. The only problems are "GIFs with Sound" (actually videos) are a premium feature and there are file size/bandwidth restrictions, but I could look for workarounds if needed.
I went with the last mounting option I showed. The way I built the chassis didn't leave room to mount it upside-down for the ghost trap idea, since the middle skewer the bearings are on supports the bulk of the model. I glued, wired and soldered the LR44 pack to the PCB and punched holes in the roof to guide the wires for the light bar under it, which were then soldered to the pads that control the LEDs after improving the contacts on the pack. The light bar had to be resoldered since the wires came loose when I pulled them through the holes, and I'm currently unsure where to wire a resistor for them. My other, wider PCB has its own built-in resistor that looks to be 300 Ω. Another screw was added where the board's screw hole is to keep it down without strapping it.
To my surprise, it can actually clear the rear entry of the Take Along Sodor Engine Works - which, amusingly, very faintly resembles the GB firehouse but if the garage were side-entry - even with the added height of the battery pack. While it's too long to be parked facing the entry, it 'can' be parked facing the narrow side where the blue locking tab is. Crudely edited to remove original markings for illustrative purposes.