If the motor is going to be ejected, then you just need to put a tiny bit of tape in it, if you even need that, to get the motor to stay in the mount until it is ignited. Thrust will then keep it there until the ejection charge pops it out like any other motor in the Mosquito.
For an MMX Mosquito, I build a clone of a regular Mosquito (or build one from the Estes kit, if you have it). For the mount, I use either FlisKits EMK 6-5 or else build my own out of two CRF 2.5-6 centering rings, a BT 2.5-0112 motor tube (or a 1.125" section of BT-2.5, and then either an EB 2.5 engine block, or else I just block off the tube just ahead of the motor far enough forward that when I insert the motor, 1/16" to 1/8" of it extends out of the back of the tube. I do not leave the forward end of the tube open, in either case. Then I assemble the mount and glue it into the Mosquito. I use either one of FlisKits' micro launch lugs or else a section from a plastic coffee stirrer as a launch lug. I could also use a standard launch lug, as called for in the Mosquito's plans, but that would add weight and drag.
The MicromaxxII motor will boost a standard Mosquito to something like 120 to 170 feet, which is plenty for that little guy. If I was feeling adventurous I might try downscaling a Mosquito to BT-4 size (0.448" OD), but no smaller than that. At that size, it might go as high as 200 feet. You could even try downscaling it all the way to minimum diameter (BT-2.5), in which case it would be 2.025" tall with a fin span of 1.3" :shock: ), but good luck finding it again once you launch it.
The FlisKits Big Honkin' Rocket (or BHR, pronounced "bear") is a minimum-diameter Micro To The Maxx kit that is similar in concept to the Estes Mosquito, 220 Swift, Centuri Two-Bitz, etc. but has its own unique shape and is slightly longer than a downscaled Mosquito. Because of the greater than average potential for loss, the kit includes parts to make three BHR's. So if you lose one, you're covered. But if you decide to drag-race them (and really, who wouldn't be so tempted?), then you are on your own.