If you want to cheat, and its OK to have the bubble at the very end,
you can use a standard brake-flare tool and only do the 1st operation
(the one that creates the bubble).

PS you can also use this method to create bubble flares with a standard
brake-flare tool. After you make the bubble, you run the cone down just
a skosh to flatten the 1st part of the bubble (the part closest to the
end).

Dave