The optimal size for a swarm trap is about the same size as a 10 frame deep. I have primarily been using a double stacked nuc, 5 over 5. I put frames in the top and leave the bottom open. I STINK at catching swarms in traps, but talked to the state inspector for our area and I think he told me the problem...

Like you, I've been putting foudation in my traps. He said he uses only old brood comb and frames with starter strips.

The scout bees actually measure the hives to make sure they are big enough and apparently the foundation taking up space is a big deal.

I just started reading the most interesting book on beekeeping I've read. It's called the Honeybee Democracy and it is supposed to be primarily about how to catch swarms. I'm not far enough into it to share any good details.

If my people, who are called by my name, will humble themselves and pray and seek my face and turn from their wicked ways, then will I hear from heaven and will forgive their sin and will heal their land. 2 Chronicles 7:14