[MapProxy] define extent for existing cache

[Volkmar Herbst]

Good morning.. 
I have a problem to define a bbox for an existing cache. The cache in in fact a prepared mbtile file so the source is empty. I do not want to create another cache but use the mbtilefile directly. The problem is that Apache throws 500 error messages when a tile cannot be delivered. Is there a way to restrict map proxy to a box for the cache?
Thank you 
Volkmar