So I build a rail that's not a fat tube, but it's a start.
6" x 10ft PVC
put the 4x4 supports in the bottom of 2x10, then,
you make a crib for the pvc to lay in like this- |_|
with 2x10 on bottom, and 2x3 on sides long ways up
screw from bottom and sides into PVC
finish the base and cross supports (from the basic tutorial in this cult)
and you're done!
oh yeah, and paint it