Kasiner (usually pronounced KAZner) used single piece body extrusions for their smooth side and fluted side streamlined passenger car kits. The fluted side car shells very accurately modeled Pullman's 1940's design, and to a degree also the fluting ACF used. Kasiner did not make Budd body extrusions.
Mac Shops in Florida had single piece Budd body extrusions and kits with heavy, cast soft metal ends. Otherwise, they built out much like a Kasiner car. Herkimer Tool also made a Budd body extrusion along with with the Kasiner smooth side bodies offered in the 2000's. I don't kmow if Herkimer used the former MacShops Budd extrusions, or a different one.
Midland took a different approach. Their "California Zephyr' style Budd carbodies were assembled with two extrusions - a roof with upper sides and a floor that included the lower car sides. The window area between them was an aluminum strip for each side that was fitted into slots in the roof and floor extrusions. These strips had the window openings punched in them for the accomodation being modeled. The carbody ends hold it all together.
S. Islander