The best advice for PenguinList is to do this as easiest as possible the first time around and not fiddle with top holes, screws, plugs, tape or hot glue/wax because he is going to use the Durchstich method to refill. Top holes can always be made at a later date if needed --- and top holes are not required, although they can speed the process of flushing.
My initial goal was to help him prevent clearance problems with cartridges having a sealed top hole. The problem could be caused by the type - specifically the height - of the plugs Mikling uses in his photograph of his reconditioned cartridges. Too much hot glue/wax could also present a clearance issue.
Knowing this issue, the next logical step is to ask Mikling if he would sell empty cartridges that haven't been tapped yet. PenguinList can flush the cartridges without a top hole. Flushing can be done with or without a Durchstich hole. PenguinList could reasonably even skip flushing the dye-based cartridges.
Another option is to buy the OctoInkjet low profile silicone plugs that I linked to in an earlier post to seal top holes. OctoInkjet also has empty but unflushed and untapped cartridges PenguinList can use. Now it might be that Mikling may have a new plug - low profile? - for sale soon. Unfortunately for PenguinList, he must make decisions based on what is available now.
Hot glue/wax had already been mentioned. PeterBJ has added the suggestion of aluminum tape, commonly used for HVAC systems.
Of course, PenguinList could just purchase a new set of Canon OEM 220/221 cartridges, swap chips, and tap Durchstich holes after using up the OEM ink. However, this will not provide him with a second set of cartridges PLUS chips.
Thankfully we do not complicate this discussion further with vaccuum refilling, rigging up a Freedom Refill contraption, plastic balls (huh?) or some other outlier method.