Someone was looking for an alternative to ready-made passive monitors. I can't find the thread right now and I figured a new post might help.
We have a thread with alternate passive monitor ideas from Lou Sorkin and P Bello here.
You can read about some popular passive and active commercial monitors in the FAQ on bed bug monitors. There are instructions linked from there for making a homemade pitfall/interceptor style monitor and to instructions for a homemade dry ice (active) monitor.
Note that active bed bug monitors may not work as well in occupied spaces, since they have to compete with a live human.