How about using a figure of a guard, or a parcel box. It won't look out of place, but you might be able to see it yourself, from a suitable angle.
Had the the same problem when building a goods shed.
You'll have it built in no time Bodgit, well, just caught up with this, looks like an interesting project, and i'll be keeping an eye on it. Good luck.
Jeff
Hi jonathan, have a look at fox transfers, they'll have something there
http://fox-transfers.co.uk/virgin-mk3-coach-hst-trailer-car-lining-detailing
Jeff