now that i'm awake, here's the secret weapon...
go to a hardware store, and look at their selection of replacement 3 prong grounded ac plugs...like what you'd plug into the wall.
you're looking for the kind where it has screws going in from where the blades that go into the socket are into a rubber (?) boot.
the boot is the secret weapon...if ya discard the useless plug part, you should be able to screw the boot onto the threads of the horn driver.
stick it on there, put an appropriate sized hose clamp on, and now ya have a reverse nozzle that you can use to stick the hose to your beak with...just stick it thru the hole where the 3 conductor wire is supposed to go, and use the strain relief clamp that comes with it to make sure the hose stays in place.
that's it. works great, under 3 bux for the part.
that said, i reccomend if you can't run a separate amp for the talkbox driver, make a simple a/b box for the speaker output of your amp.
all ya need to do is swap the hot wire...the grounds can remain connected..
from amp out tip to center pole of spst switch....from pole 1 to hot of jack one, from pole 3 to jack two..
connect the grounds together, done deal. plug the talkbox in one side, and run a speaker cable to your amp speaker on the other side.
i know some people are gonna freak, "you can't do that!! you'll blow your amp!!"
but you won't. remember, it's an impedance, not a resistance. if ya hook up a fast meter to your speaker output while playing, you'll see the dc resistance go all over the place with say an 8 ohm speaker...including NEGATIVE...where the speaker's not transducing, but conducting. this doesn't hurt your amp.
if the switch were to fail, it could...but after doing this for 30 years with the same box, i've never had a problem. if concerned, use a dpdt switch and wire both sides of the switch in parallel as a failsafe.
btw, to MY newb eyes, your schematic looks pretty good...but i don't think you'll get enough drive out of a 386 to really drive the talkbox well enough to really be useful. remember, you can drive a 1000 watts into a 20 watt speaker with no issues, unless you clip the power amp, or completely drive the speaker to the point where the basket gets blown off the spider and the voice coil melts... that's not because the speaker couldn't handle the power alone, it's from pushing the speaker harder than it could survive.
my point is, better to have too much power and turn it down, than to have not enough and try to make up for it later.