"schoey2k" wrote in message
ups.com
Hi,

What would be the effect, if any, of mounting a tweeter
(O/D: 130mm; Voice coil diameter: 25mm) directly behind
an aperture (76mm diameter and 16mm thick)?

Reflections and resonances in the area between the edge of the hole and the
edge of the diaphragm.
A sort of a cavity effect due the the excessive thickness of the hole - a
resonance followed by a roll-off.

Far better to front-mount the tweeter.

This is an absolute last resort but due to the makeup of
the speaker cabinet there is no way to secure it from the front.

If good hi-fi is not the goal, then whatever. There will be "some kind of
highs".