One of my favorite Anglie's buildings carries a ghost ad for Texas and Pacific "23 Hours to St Louis Free Reclining Chair Car". However, Angie, unlike Kingmill, puts its image directly onto a sheet of foamcore. I find that a paper sheet attached to foamcore is in fact likely to warp, so I like to build a more substantial backing for either Kingmill or Angie's.
Not only do top, side, and bottom pieces stiffen the front, but I put an intermediate horizontal piece inside, backed up by the rear wall, to stiffen the assembly further. So far, I haven't had warping problems with Kingmill buildings roughly this size if I build this type of backing. However, I wanted to add this Anglie's building to a larger Kingmill flat I did several years ago. I found the amount of stiffening I added to the larger Kingmill did lead to a certain amount of warping, but it's still acceptable for a background building. Here are the two assembled into one piece: My experience with the slight warping on the building to the left led me to use the more substantial form of backing that I now use on the Angie's T&P building.I've also found a supplier for ghost sign decals that will fit on DPM type buildings, T2 Decals. You can get them most cheaply on Amazon. I do find that they're quite hard to get off the backing, and in fact on the set I got, I had to soak them in water for about half an hour and then use an X-Acto blade to peel the decal away. But it finally came off. Here are two examples on model buildings:
Finally, Blair Line makes business sign sets for DPM type buildings. A couple of sets have lasted me many years. Here's a sign I just recently added to a 25-plus year old DPM building: