Frank,
I am new to this forum and started keeping bees in hTBH (horizontal top bar hives) last year.
It is best to ask in your area when is the best time to set up swarm traps. I will do it in the middle of May because of the Swedish law, otherwise beeks in USA do it already in April.
I have build a few bait hives the same size as my long top bar hives just a 12 top bars lenght. Bees dont prefer cavities larger than 60 litres (Seeley) so I settled on 12 bars lenght. This is my first year to bait bees so will report how it went. Maybe there are no swarms in my area.
The entrance is a drilled hole 30mm, because Seeley's research shows that bees prefer it.
Place the bait hive on a tree, shed, or what ever 2-5 meters above the ground. Make sure to staple a piece of chicken wire at the entrance to keep away birds from nesting in it.
Dont forget to prime the top bars with wax stripes. Like this bees are encouraged to build streight comb.
I can assist you but as mentioned Im not an expert.
Books which helped me are
Top-bar Beekeeping by Les Crowder (/practical stuff great hive maintenance illustartions)
The Barefoot Beekeepr by Phil Candler (phylosophy on top bar beekeeping)
The Buzz About Bees by Jurgen Tautz (up to date bee biology)
You can also find lots of info on biobees.com and beesource.com (both forums)
On you tube I would highly recommend to watch the videos published by OutOfaBlueSky on you tube;
http://www.youtube.com/user/OutOfaBlueSky?feature=watch
My blog on beekeeping;
http://cheguebeeapiary.blogspot.se/
