My son and I built a top bar hive nuc box which we will start using as a swarm catcher. The plans and the cedar top bars are from Beethinking, but there are plenty of free plans on the net, and the concept is not difficult.
The wood is salvaged pallets, mostly pine. We had to glue up 3 and 4 pallet boards to get the full-size panels. Wood is cleaned up and planed smooth. The wood is free, the workshop equipment we already have, and the only additional things you need are some waterproof wood glue and a few stainless steel screws. The box fits 8 top bars. The volume of the box is about 20L, maybe a bit smaller than the ideal 40L that swarms seem to prefer, but it should do for a small swarm. Baited with clean beeswax (painted onto the top bars) and a drop or 2 of lemongrass oil.
Safety tips: We choose only HT (Heat Treated) pallet wood. If you are planning on doing this, make sure you never use pallets stamped MB (Methyl Bromide) as these are toxic. If you are using power tools, make sure you double check that you have removed all nails. We use a metal detector (stud finder) just in case. A circular saw hitting a large hidden nail will cause kickback and potentially serious injury.
Finished box with lid
Cedar top bars in place
Cedar top bars removed
Next project is to build a full size top bar hive. Its the same design, just 3 times longer. If/when we catch a swarm, we can then transfer them to the top bar hive.