It is much easier if you start with boots that are as close as possible to being right.
Obviously, you are best off getting new boots that fit as close as possible. We have just covered going to the right people.
Athletics training text advise that you never wear used footware, because moisture makes them good ways of carrying infection. If you buy used boots, be sure to spray in a disinfectant (e.g., Lysol), and an anti-fungal spray designed to disinfect athletes foot. That won't cover everything, but is a good start.
Making boots is not an exact science. No matter how carefully you are fit or the boots are made, custom boots often fit poorly. So the best bet is to go to a store with a large selection, and try pairs to find one that fits well. In fact two boot pairs of the same model and size will not fit the same.
Only if nothing comes close should you consider custom boots.
Rental boots are used boots. It is not possible to do most of the types of customization discussed in this document. However you can still create your own insole. Be sure to remove it before returning it. Personally, I would also want spray in disinfectant and anti-fungal spray. It is often possible, if you get to the rink early, to request a specific numbered pair, so you can consistently get the same fit.
Rental boots are always ordered quite wide, and they see fairly heavy use, so they differ pair to pair.