Trim up your pieces of pipe so that the straight sections of plain pipe are about 5 1/2" long, with square ends. Then use the penknife to trim all of the flash and stray stringy bits off of the pipe, couplers, and endcaps. Use the sandpaper to round off the inside edges of the pipes and the male couplers wherever they might come in contact with the film to prevent scratches. If you think you can bevel the inside lip of the male couplers smoothly, go for it. Once all the trim and sanding is done, painstakingly wipe everything down to collect the plastic dust, lest it become embedded in the emulsion during use or cause scratches on the base.

To each female threaded coupler, glue on an endcap with the ABS cement, following the instructions on the can.

To each pipe section, glue on one endcap.

Glue a male threaded couple to the other end of each pipe section.

Let cure, then wash them thoroughly, then dry. You should now have 4 tubes, sealed at one end, and 4 threaded cups that screw onto them.

Why go to the hassle of using glue-on endcaps glued to threaded couplers instead of simply using threaded endcaps? Volume! You can put a couple ounces of chemi in one of the threaded cups to make processing easier and more consistent.