Hi stuartew,
You mentioned that you looked into decals. This would be the best way to approach what you are doing. The material applied to the wall vs a decal can be seen below.
Follow the below tutorial on how to quickly setup a decal in your level.
It’s normal for materials to tile like this, it’s actually a feature. Look up ‘tile-able textures in games’ for more info.
The other option would be to create a single plain and apply a material to it. Take a look in the Reflections demo on the Marketplace for an example of this in action. The posters in the level are done this way.
I hope this helps.
Thanks, TJ