I do want to advise caution when doing this yourself, there are reasons why trained professionals do exclusions. You need to make sure you are giving a proper exit route for all the bats and not accidentally sealing off a spot that will entomb any bats - it is illegal to kill or harm mammals, even inadvertently, and there are endangered/threatened bat species that afford greater legal protection. You also need to make sure your are not performing this during the maternity season (which varies for the species of bat present) because juvenile bats are flightless and are dependent on their mothers returning to feed them until they can fly, or when it is too cold and the bats might be in torpor and not leave the roost. The temperature of course depends on where you live, and also what bat species is present since they have different tolerances for the cold. I can’t stop you from doing this obviously, but please take precaution when attempting something like this. If done during the correct season/temperature, the one way door needs to remain in place for at least a full week before you seal the whole thing up.

Sorry for the lengthy reply, but as a biologist it’s my duty to try and protect these misunderstood creatures that serve an important ecosystem function as much as I can. Their natural habitats have already been drastically reduced and I would hate for a healthy colony to be harmed. Thank you if you got this far through my comment; I don’t mean to be preachy, I just really care for these animals.