I tried my Kenwood under the seat but the limited range is no good....the most important time that you'll need to be able to speak to others is when they're out of sight..plus when leading rides and wanting to speak to the tail marker (Or middle man to relay it down if lots of riders and a long trip) 500 mtres just ain't god enough.
I've gone for a different approach......I got a cheap security box (under 7 quid on sale IIRC) and I drilled it and mounted it on my GIVI mounting plate on the rear carrier, then the GIVI box goes neatly on top if I need it to.
The box before the install;
During;
After install with Givi plate back on top;
Givi box and Toerag bag remounted;
And the box opened up;
Once it was in place, I took a power supply from the rear light circuit to the box via a waterproof gland, and ran in the Autocom lead through the same gland. (Autocom lives under sadle and is wired in to battery via fuse)
I then drilled a 20mm hole in the back of the box and fitted another gland....then the Kenwood goes in the box and is locked securely away but the aerial sticks out the back through the gland....That gives me good range and I can change channels easily by opening the box for access to the radio.
There's also enough room in the box for cable ties, insulting tape and other bits and bobs that I want to stash away.
I can't photo the kenwood in situ 'cos Bakerman's got it at the moment, but it does work well and it's a neat solution IMO.