We had the original Webasto heating fitted to our single aft cabion B37 from new. It was installed in the lazarette near the centre and to stbd of the rudder post. This allowed the exhaust to exit through the transom and it did not take any space out of other lockers.
Access is OK through the hatches at rear of cabin or cockpit locker. Has worked very satifactoriy for last 4 years.