I heard of method using a plastic pipe with one end connected to steamer and on other end is cap with a hole. Hole is just enough big to allow excess of steam to freely flow outside.
You put whole bow into the pipe and turn on the steamer, that's it.
Main problem is of course, you have to have steamer which I am sure not lot of people have.
Possible alternative is pot of boiling water which instead of being completely covered on top, has special cover with hole on which flexible tube is mounted. Other end of that tube is connected to the pipe mentioned at beginning. Thus you got a relatively cheap steamer.