Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules
night fighters normally preferred to attack from below
A turret on the bottom might seem like a good idea, but I doubt the gunner would be able to see anything below against the dark ground.
I've read the German night fighters like to approach from below even before they had the slant music guns, that way they were invisible to the tailgunner , and the bomber would be visible to them against the night sky sometimes. They'd start a gentle climb from below, and fire, ending up in the tailgunners lap before he could react.
Luftwaffe attacks on RAF Bomber Command aircraft numbered in the 10s of thousands. If the tail gunner fires only 10% of the time that's still enough to make a night fighter pilot nervous. Especially since there are plenty of first hand night fighter pilot reports stating the tail gunner shot at them.
One tail-gunner told me that his captain had told him, that, if he opened fire, he'd have him court-martialled; he was only to warn him, so that he could take evasive action. I doubt that captain was unique in his feelings..
The ideal US bomber for RAF Bomber Command would be the B 29. A big bomb truck with near Mosquito performance it would have been a very tough target for the majority of LW nightfighters.