i'm at the same stage as you, building the carriage to hold the saw now, dont have the $$ or space right now to build a full on trailered frame type mill. I was thinking of ripping 12' 2x6's in half, then dadoing slots into the edges, and using 4"x12' plywood in the center, sandwiched between the 2"x3"s, similar to engineered floor joists used for building homes. i thought this would provide ample stiffness, while still being light and portable. i plan on staggering the center plywood piece 6" in on one end, and 6" overhang on the other, then i could slip 2 12' assemblies together for a 24' rail when needed for longer milling. seems like it should work ok, i'll let u know when i get there!