Well, if it's for free, that's different.
There is no really good place for a unit of that size given the constraints you've outlined. The tail is the best place -- protected, accessible and convenient for getting switched +12V from the tail light assembly. If you try to locate it in the tool tray, you'll not be able to remove the tray to get access to the ECU and battery. There's no room in the relay box and that is inconvenient to boot. Either side of the relay box is not large enough to accept the Centec. There's no place under the tank large enough either.
Maybe one of the pockets of the fairing, but you'll run into the same problem as with the tool tray come the day you need to remove the fairing.