Use this enclosure for an outboard RPI Relay Module connected via a 40-pin GPIO cable.

Cutouts are provided for three snap in 110V convenience (US standard grounded) outlets, a standard strain relief and circuit breaker.  

The hole for the strain relief will accommodate cable with 16 gauge wiring which provided you keep the cable length to a meter or less is adequate for 15 amps of current.  The 18 gauge wire I used is rated for 10A at the same length and is the rating of the circuit breaker.

I highly suggest that you make use of the crimp on ferrules for the relay contacts as shown.  They make an excellent, best practice connection regardless of the wire type.  The convenience outlets used here have individually tinned wire in the stranded cables making the ferrules a necessity as tinned wire of any sort tends is not suitable for these connectors.  The “wire nuts” are crimp on end terminators.  

The heat pressed (a micro butane torch is ideal) holes are sized for M2.5 x 3.5 x 4; the standoffs to hold the circuit board are M2.5 x 20mm.  Screws used are 4x M2.5 x 7 and 4x M2.5 x 5

I printed in PETG and the 2mm thick walls seem ideal with components holding (and removing/replacing) securely.