Originally I used a piece of custom cut plywood to support the coin door hollow whilst I was applying the decal, but then found the easiest way to eliminate the sagging in the middle was to simply precut out the blank white vinyl in the centre of the decal before applying. As with the siderails, you have a marginal safety net with the inner rectangular edge as the lip of the coin door extends beyond the cutout piece.
Then it is just a matter of doing it in the same way as the sides, cut off half an inch of the backing paper along THE BASE (always start at the base to make sure the artwork stays horizontal, not left to right or vice versa) and then work your way up removing the backing paper an inch or so at a time.
As before, the main concern is making sure the artwork is correctly placed before applying - for example that the winged logo sits below the footprint of the coin door once it is installed.