I think it should all be just where the placement of the building is in the build menu. This would involve moving some placements around but I think it would be the best way to go. B+Q for Tower, B+W for barricade, B+E for Energy wall, B+A for Aura, B+S for book shelves, B+F For Factory, B+C for Command Center, So That leaves D,Z, and X for Lumber mill, Merc Tent, and Item Vendor, I think B+D Would be best for lumber mill, and the other 2 could be anywhere.
Something like this: http://imgur.com/Uj3tfJb
Edit: B+Q Could also work for lumber mill swapping lumber mills and towers.