Catching swarms is fun, but chasing them down and gathering them up isn’t always an option for everyone. It’s so much easier to just let the swarms come to you as they move into a swarm trap. It can also be challenging to prevent our hives from swarming, so giving them an option to move into will make it much easier to catch our own swarms.
I recommend reading Dr Tom Seeley’s book Honeybee Democracy where he writes up his experiments to better understand how bees choose a home. Within that book you’ll learn about ideal cavity size and entrances.
The dimensions he gives are a cavity between 30 & 40 liters and an entrance no bigger than 15 square centimeters.
Some beekeeping supply store will sell you a “swarm trap” made of pressed fiber in the shape of a flowerpot. I really don’t recommend this option. Nobody has a hive that shape, so moving the bees from the flowerpot to the hive is tricky. Instead, use equipment that matches your hives. If you have a top bar hive, use a trap that has top bars that fit your hive. If you use Langstroth hives, use a deep (about 40 L) or a medium (about 30 L) with frames so you can easily move the frames from the trap to your hive without having to cut and string up comb.
A 15 cm2 opening would be a hole with a 1.72″ diameter. So a hole anywhere between 1″ and 1.5″ would be perfect.
For bait, a couple drops of lemongrass oil placed inside the hive, or on a cotton swab is just about all you need. You can also include some old brood comb for additional attractant.
Sometimes bees will move into just about any old equipment, but by following the basic principles above, you can improve your odds of catching swarms the easy way.