They're officially gone! Today I removed the last school bus flashers. It was literally a matter of a few seconds work.
Measuring the holes, it seems the apertures are 5.5 inches in diameter. My fans are 4 inches in diameter. Clearly I'll have to do something other than just sticking ventilation tubing through the holes. That's not a huge problem though. I have some spare sheet steel I can use to cover the holes that I can also drill or cut a smaller hole in.
The aim is to put a ventilation fan on each side, powered by rechargeable batteries. Those will be charged by one or two solar panels in all probability.
In the store, I picked up some mosquito mesh, some plastic tubing and some plastic drain covers. Those covers should support the mosquito mesh adequately. The fans can simply be screwed down over the lot.
Looking at the front door, it still slips outside the receiver. I tried adjustments but nothing so far has worked and continued to work beyond a few days. The solution might well be to add some kind of guide - possibly at the bottom of the door. Maybe this might be a use for some neodymium magnets?
I had wanted to use the school bus flashers as repeaters for my turn signals and brake lights. That turned out to involve extra work at a time when I just want the construction to be completed. I'm hoping my ventilation fans will resolve the excess internal heat in the bus.