As I noted above (as has Cris), the Sonic Igniters, which are made in Penrose and sold for use with the relabeled Aerotech motors Estes sells, will work. Here is one installed in a Q-Jet B6W (which has the narrowest nozzle of any of the 18mm Q-Jets) and another lying alongside showing how far up inside the one in the motor is.The nozzle opening on the q-jet is very narrow. Also, you have to ignite them from the forward end. That means the starter has to be able to pass through the narrow nozzle and then several inches to the top.
None of Estes starters will fill that role. You need to use the starters that come with the q-jet motor.