Takes a newer style 4 channel relay board commonly found on ebay, and securely screws into a carrier which should be fitted with 8 brass inserts for m2.5 bolts - 4 top, 4 bottom. These then allow the din mount to be screwed one side, and the relay and case screwed all together to the carrier.

The side of the case to fit an sp13 ‘aviation style’ 6 pin connector, but could also just allow access to the header of the relay.