Those people who are lucky and maybe have talent like me, will be more exposed to many things that are taking place in a new and old BHs.
The more BHs you visited the more you will learn about those swiftlet.
One observation which I will normally focus upon during every visit is to observe where will be the birds favorite locations to erect their nests.
If you can list them in a proper manner these will be their most likely places:
1) 90* corners. Their favorite and their is a very strong chance that until all these 90* corners are occupied only then they will start building beside it.
2) Above those internal sound tweeters. This seems to be another favorite location. What can be the very possible reasons? You need to know that when they entered your BH they will not be able to see with their naked eyes. Your BH is pitched black and the only thing that they can use is their ears. They will fly towards the tweeters. These tweeters, playing internal sound, will become their resting place. Once these couple are ready to mate the male will start searching the nearest location to the tweeter.
3) Fake Nests. Ar Pedas BH for example nearly 90% of the nests found inside it were on those plastic fake nests. About 4-5 were directly on the planks corners.
There are a few more but the 3 above seems to be their most preferred locations.
If they like these places my method of approach is to create them and not covering them.
If they like those 90* corners I would like to recommend that let them be opened and not covered with those corner cover woods.
If they like to erect their nest above those internal sound tweeters increase the tweeters number.
If they love to erect on those fake nests install as many as you can.
You should let them have those things that will make them happy.
You must remove or restrain yourself from installing something that make their life difficult.
Each of the three items mentioned to me are like those baits that we put on our fishing line.
Yes if you happened to love fishing you know that the best way to catch those fish is to hook a bait on your fish hook. If you put no bait the chances of hooking a fish is slim.
So why must you install those corner covers when you know that those swiftlet loves 90* corners?
Populate your nesting rooms first.
Once the nests population are adequate you can then look at methods to force those birds to erect their nests onto the straight planks.
Think about it and try not to cover those 90* corners.