I would expect the ballots to be checked for validity wherever they are being counted, so fakes would be weeded out then: they would have to match someone on the electoral roll. And provided they are in sealed in envelopes, which I think is a legal requirement, they are not going to be easy to tamper with and not have the tampering detected. I suppose the miscreants might have a huge stack of official ballot papers and only put ones for the GOP candidate into them for delivery, discarding all the ones for the Democrat, but there is no way anybody could fool themselves or anyone who saw that going on that it was a legitimate activity, so there would be a distinct risk of being caught.
I think their thinking is that since Dems are pushing for mail votes and Repugs are pushing for in person votes, that any drop box would be primarily (overwhelmingly?) Democratic ballots. Therefore if they just dump the ballots into the nearest river or incinerate them it will cost Biden a lot more votes than Scrotus. 🤷🏼♀️
The only other way for them to cheat with 'fake boxes' would be if they had copies of the electoral rolls and spent the time to compare the sealed envelopes and discard the registered Democrats' envelopes. At least in California, the envelopes have the name and address of the voter on the outside.
Deep into tinfoil hat territory here. At least I hope I am! But we're taking our ballots to the Registrar's Office just in case.