These are parts to help with using both standard PVC and a non-standard part to make the center tubes (the beam line and filler tube) as well as the injector tubes.
The Beam Line is the tube that connects to the center cover in the middle. It's the one that gets a brass fitting at the top. In the PakRatJR parts, it's called Tube_Short. If you want to make this out of PVC, get a 1" PVC pipe and cut it to be about 3.91" long (about 99.5 mm) and print out the ShortCenterTubeCap to go on top. Also print out a CenterTubeAnchor to anchor it to your center cover. The spot for the beam line in the center cover is wider than the outer diameter of the 1" PVC, so it's helpful to have a way to attach our 1" PVC more permanently. Drill a hole right in the center of the round spot where the tube fits into on the center cover using a 5mm drill if you have it (or a 3/16" drill for a tighter hole, and a 13/64" for a little bigger) so that you can get a M5 bolt through. The M5 bold should be about 20mm long. The Anchor accommodates both a nut and washer for an M5 bolt, so insert those into the anchor using the hole on the side and then put the bolt through from the center cover through the anchor to secure the anchor in place. Now you have an anchor you can fit the PVC pipe around. I recommend using an epoxy to epoxy the beam line to the anchor to help keep it firmly in place.
The other tube on the center cover is the Filler Tube, or Filler Plug. The Filler Tube is called Tube_Long in PakRatJR's plans. Again, this can be replaced with 1" PVC if you'd like, even though it has a slightly smaller diameter. Just cut your 1" PVC to be about 4.9" long (about 124.6 mm) and print out TallCenterTubeCap to go on top. Then print out the CenterTubeAnchor and drill about a 5mm hole (using either a 3/16" drill bit or a 13/64" drill bit, though if you have 5mm that's best) right in the center of the space where the Filler Tube goes. Then put a nut and washer in the anchor and, putting a M5 20mm long bolt through the hole you drilled going up, screw it into the anchor. The PVC pipe then fits over the anchor and should be secured in place using epoxy or glue of some sort.
The injector tube pipe is a little harder to find, it's an Everbilt Polypropylene Flanged Strainer Tailpiece with the UPC of 041193461223. Just search for that at homedepot, it's the perfect size for the PakRatJR injector tubes. Just make sure you find some really straight ones and print out two InjectorTubeCap parts to go on the ends. They should be cut down to the correct size, about 6.6", or 168 mm, which allows for the 3.2 mm caps to stick out and finish up the length. Make sure you use the side without the flanged end so the tube has a consistent outer and inner diameter the whole way.
UPDATE 6/2/2019: Re-uploaded CenterTubeAnchor to better fit a 5M washer. It may be necessary to clean up the washer side of the print because I recommend printing with no support, and this can cause some strands of plastic to droop down into the washer's space.