Here are a couple refinements to the kindergarten sketch-
Press-
http://tinyurl.com/98w3pThat candystriped support at the back could be a fat dowel or
whatever, attached to the base to stiffen up the guide bars.
Holders-
http://tinyurl.com/ccpp3It could maybe be less complicated than that and still work (paper
theory again). The front, drill holding part of the standoffs could
just be pipe clamps or a metal strap. The holders could also maybe be
cut out of a single triangular piece with the drill mount hole already
in it, then you could just saw the front off to get the two pieces.
Also, all the pulley crap could just be large screw eyes.
PS- I've probably spent more time typing and sketching than it might
have taken to make the damn thing. ;)
-George