To get a PO Box with Deutsche Post you have to have a street address in Germany, and they expect you to empty your mailbox at least once every seven days - so this would not be suitable for someone who doesn't live in Germany.

There are various private companies that provide mail forwarding services - try a Google search on "Mail forwarding service" +Germany (I just did)

A word of caution... I don't know what sort of project this is, but we discovered that just having a postal address in Germany won't necessarily work. We tried to get a German VOIP number using a relative's address and our application got rejected because we were not listed at that address in the population register. You may well find that "agent" means "real person" as opposed to "accommodation address".