3D print both parts, top requires support, the baseplate should be fine depending on the printer.

Solder to the sensor board, place the lens in the center rectangle then place the sensor on top of the lens making sure that the sensor has the yellow kapton tape removed and the sensor sits in the small circle on the lens

Solder to the middle and 1 outer pin of each microswitch (each switch should have one pin to ground and another to the arduino) Solder at a right angle to the switch so it fits easily.

Both switches line up on the rectangular pillar in the front of the mouse, use 2 M2 machine screws and nuts to secure them as seen in the picture.

The top shell prints with built in support material on the left and right of each mouse button. It is a very thin line of material, take an x-acto knife or similar razor blade to cut it out. Each button should move without clipping the rest of the mouse body

Slide the top shell over the baseplate so that the tab goes into the shell's front slot. Then just secure with an M2.5 machine screw.