Allright I see.
Here´is my idea of a modular longboard made from scratch. Bad drawings but just threw something together..
Material is Flexlink Alu 30*30 beam and 3d printed outside houseing for a nice look. Maybe birch plywood enclosure top and bottom..
Benefits is that it´s modular, cheap, light, minimal machining. Add 2 springs and a bolt on the slot where the motors are mounted and you got a belt tensioning system as well.