I agree, it might be a little OP to protect a player, for those who would abuse it at least.

I have a few suggestions, but it might not be enough:

Perhaps there can be a building list. Meaning BOTM entries are supposed to be finished; so there could potentially be a second list every month of who is planning to submit the next month. This would give a player 1 month to prepare; and thus only those on the prior months posted list would be given any sort of protection. (This would not restrict people from submitting, just give people a chance to publicly announce a upcoming build and thus seek protection).

Another idea is to give BOTM builders a specific rank (also possibly for a set time period) that would enable and disable certain features. For instance, while assuming the Builder rank, you would be protected from pvp, as well as seige, but you would not be able to attack either and certain other things owuld be disabled like seige, and other commands that would be abused. You’d get protection but at the cost of being pretty much unable to do anything BUT build.

I’m not sure either of these suggestions would be any less abused, but maybe it will give you an idea that won’t be!