Given the obvious political climate today it seems insane to have your opponent (or their administration) in charge of your safety.
My solution would be to allow all former presidents, as well as those running as non-incumbents, to provide their own security at the expense of the tax payers.
As with most things, it could likely be done at a cost savings by private industry and of course would remove any chance of it being “mishandled” intentionally for political gain