I've installed some new ceiling register boots to use with flexible ducting and realized after install that they have a smooth connection with no lip. I'm assuming they are made for metal ducting, as I think flexible requires a lip for a tight connection. How can I securely attach flexible ducting?
One idea I had was putting five or six screws around the connection to act as a lip. I also thought maybe there is a tool that I could make some dimples that would do the same thing. Is a lip even necessary for a solid connection?
UPDATE: Since I don't really know the proper names for things, I think pictures might help.
Here is pretty close to what I have:
Here is the lip I'm talking about that is on the current register boxes:
Here is one with no lip:
On my current ducts, there are two zip-ties, one on the inner core and one on the outer insulation. The inner core zip tie is behind the "lip". I assumed this was necessary for a solid connection (Note: I'm using mastic, not tape. Whether one is better than the other is another topic). Here's a picture of the inner core I was referring to that I want to put the zip-tie on.
So I've now changed my question to: Is that lip needed and if so, any way to fake it if not there?