The Flasher plate can be screwed (or fixed using double-sided tape) upon the Heng Long BU without need to solder or anything: you re-use one of the top screws of the BU (this is detailed in the manual).
The original BU still plugs into the normal turret connector, while the flasher has its own wire to be plugged into the I.B.U.
This way (this is the older version of the flasher, now it's circular):
UPDATE **: please find the new manual 1.2 attached