I've been fiddling with building a planing sled setup for a couple of months now. I use mitre slot rails on the sides and 10mm rods and linear bearings for the crossbars to support the router. I found that with too much resistance, I was pushing the router out of "plumb" by pushing too hard to move it.
I've been meaning to post some pics, but wanted to work out the kinks first. Soon.
BTW, there are lots of really good parts on ebay for this - linear bearings, guide rods and pillow blocks. You'll want at least a 10mm diameter rod, but could go up to 20mm for extra rigidity.