Here's an idea you may not have thought of yet... ;D
(https://busconversionmagazine.com/forum/proxy.php?request=http%3A%2F%2Fcc4651b36be068fed744-384adb2bca8dabe6b2947f7066085aa9.r65.cf2.rackcdn.com%2Fwp-content%2Fuploads%2F2015%2F04%2Fscholl-bus.jpg%3F58&hash=57141300a34b07f1248448c9c97018395783af43)9cf4
Sort of a modular concept. I like that idea that way if you need more space, just add on or subtract if not needed.