Print the two parts using a minimum of 3 shells and 35% infill, no raft or support required. Once printed, remove the small amount of manually added support (in the design) so the pins/lugs are cleared and able to locate into the U-shaped part.
Unload your filament before starting this so the filament guide tube is free.
Install when the build plate is lowered (i.e. after a print has completed) so the rear panel is accessible from the front of the machine. Push the U-shaped part through from the front over the metal chassis immediately in front of the standard guide tube holders... it will all make sense when you do it.
Next, push the second part into the U-shaped bracket, which will now be protruding through the rear of the machine, from the rear until it locks in place via the pins top and bottom. Make sure the larger hole of the second part is facing "up" as that will retain the filament guide tube. The smaller hole is where the filament will feed in from the bottom.
A bit of wiggling and force will be required but once in place it's on for good...
This eliminates the open rear of the guide tube holders so it is securely held in place.