Suggest using support when printing face down to keep XT60 sockets clean.
Solder wires to XT60 connectors first, then insert into the cover. They are a very snug fit and might require a bit of force to get them in.
Use hot melt glue on back side of cover to grab terminals and wires of the XT60 connectors to help hold them in place. A dab of epoxy in the bottom of the connector seat will work too but make sure it is a thin (low viscosity) otherwise the connector won't seat properly.
Tap holes in side for M3 screws to attach to the PSU.
Optionally use brass threaded inserts heated with a soldering iron into place. If you plan to do this make sure to resize the holes in the CAD file or drill the hole bigger after printing.
Measurements taken directly from PSU to make a tight fit.
Included Fusion 360 file.