Rockler (woodworking) has what's essentially a mini-gutter (about 1"x2") with adhesive back (Wire Master Channel, #13665); it is expensive ($10/32" at their retail side) - they also have some other nice specialty items like that. Companies like Outwater Plastics often have the same stuff for lower prices, though I didn't see this item there at first glance. Usually, though, I drill holes near upper and lower edges of a 1x4 every 6 to 8 inches and insert 3 to 4" lengths of 3/8" dowel, rounding ends. Finally, staple Velcro cable ties (excess holiday variety at dollar stores 4-6/$1) every foot or two at the bottom edge of the rail, and use adhesive Velcro dots to match up at the top edge. Additional Velcro ties to the dowels where wires leave the 'tray'.
However, I have seen at a dollar store a knock-off 'tool holder' consisting of numerous semi-flexible plastic loops in a row several inches long, normally holding screwdriver shafts. I didn't think at the time, but these would work very well for organizing cables, spaced every 6 or 8 inches - I'll get a bunch next time I see them.