First, find all 6 x acrylic plates and 8 x 4-40 machine screws and 10 x 4-40 hexnuts.
Peel off the acrylic backing and pop out any bits left from lasercutting.
Gently insert 2 4-40 screws into the two small holes on the front plate. The front plate is symmetric. You may need to use a screwdriver to (again, gently) feed the screws in.
Then screw the hexnuts on the end so that they are just below the end of the screw.
Place the side piece with a hole in it on the side with the end of the tube. the hexnuts should slide into the T-slots in the plastic.
Gently tighten both screws, not all the way but enough to keep the side piece in place and not sliding out.