I think wider can be done with synthetic materials. With the 5" x 5" square sections of PVC fence post covers, 3' sections could be turned horizontal and glued stacked together. A little innovative cutting would allow the bats to climb up and down through the sections.
Yesterday, I saw black ribbed ABS plastic drain pipe in huge diameters. It might prove easier to get a nusery colony of bats with appropriately spaced baffles inside one of those.
Rubicon bat houses are made from sheets of recycled milk jugs. Their current bat house is not bad looking. I would like to see one double the size of what they now offer. It would be great should someone who has tested a Rubicon bat house speak up about their experiences.

