Finding a standard project box for PLC is not difficult. The box above is available from 1.5 GBP and up on ebay. They have a standard click on system in the back for wall mounting and opening on the sides for connectors. This is quite popular within automation that mount these side by side and have cabling above and below. The illustration below show the main principle on how they are used
I ordered a few of these earlier, but gave them away because they where to large for what I planned at the time + I would like to avoid the need of loads of custom wiring. This is still an option, but I would like something a bit more plug, play & modular.
Automation engineers would love this type as they are used to it, but if you scale up modularity with rather large boxes you also need a large cabinet – I like to address less space usage and less cabling needs. We do after all plan a backbone bus that is not present in classic solution,