I tested a lot of glues, and concluded that syrupy thick cyanoacrylate glue is the best for glueing the plastic stalks to the wood keys. The "StickFast CA Thick" from Highland Hardware was marginally stronger than other thick CA glues. The keys will take a lot of pounding, so a really strong bond is important. That requires the plastic surface to be rough.

To roughen up the ends of the plastic key stalks you previously cut with a hot knife, sand them with coarse sandpaper attached to a sanding block while they are protruding slightly from a hole you drilled into a piece of wood that holds them firmly. Drill the hole on a drill press, or with a plunge router, to ensure the hole is perpendicular to the wood surface. Your goal here is more to roughen them than to level them completely, because you don't want to make them too short. A too-short stalk may not click sometimes, and the stalk will need to be replaced with a longer one from an additional keyboard.

Make a jig out of 1cm thick lucite, which will be used to glue stalks. I used a thick T-shaped ruler because that is what I had on hand. Drill one or more holes perpendicular to the surface and all the way through; use the appropriate diameter to hold a stalk snugly. Use a caliper to choose the correct drill bit for the stalks you have.

To glue the stalks, apply thick CA glue with a toothpick (or the sharp nozzle on the bottle) to the roughened end of the stalk, and a little bit (1mm) up the sides, too. Insert it into a hole in the lucite with long-nose pliers, carefully aligning it to be upright (North is pointing North), and protruding 1cm. Put a drop of thick CA glue in the bottom of the hole in next key to be glued, and spread it around with a toothpick. Put this key over the key stalk and push it down so you feel it pushing the stalk 1mm deeper into the lucite, and its base is up against the lucite jig. Now flip the jig over and use a long piece of scotch tape to hold the key in the right position. That is, the letter should be aligned vertically (North-South) in relation to the key stalk, and the stalk should be well centered in the hole, as seen by sighting through the lucite (see photo). Let them dry with the keys upside down, so glue will not run down the stalk, but pool in the hole you drilled in the key. If the lucite becomes clouded from glue fumes, be sure to sand it smooth with fine emery paper before gluing the next key, so you will be able to sight through it and check that the stalk is in the center of the hole. Probably a lot of improvements could be made to this process, but this worked for me.

Keys that are not glued properly will make your keyboard look all wonky, and may rub against the keyboard mechanism and get stuck, so center them and align them with the axes of the key stalk carefully and check alignment after taping and before walking away to let them dry. Be sure to let them dry AT LEAST 12 hours in the jig, then AT LEAST 12 hours removed from the jig for better airflow into the key, before plugging them into the base to test. Otherwise, the spring may get glued permanently to the key. [This was a costly and time-wasting error I made, ruining my first Unicomp keyboard.]