TC- A pencil, a piece of string, and a small nail make a cheap compass
(nail being located on the center-point, tie it to the pencil. The longer the string, the larger the diameter of the radius)
These traps (12" diameter) I'd made for someone else. I haven't many places where these (funnels) are feesible (waterdepth), so I stick with "standard" colony traps...so I can't offer any input on their "success rate" (sorry).
I can't see where doors would be required, much like the minnow trap (as previously mentioned).
Also, I drilled a few evenly-spaced holes in the large end of the funnel, & use hog-rings to hold it in place.
PS- You're more than welcome Hal.