Making an overhead router sled/jig

Not sure what to call it, but after making a couple decent good router tables, I thought I'd try something different. I got a Bosch Colt palm router, and I really like it for small work. And I've always like working with the router on top instead of under the table. Great visibility & freedom, etc. So... I thought I'd rig up something with rails. Not built yet, but the ideas are something like this:

- a 2'x4' surface to handle 48" x 18" or so
- a pair of small t-rails (a) running along the 4' lengths
- larger rails (b) that slide along the t-rails above, lockable with knobs
- a pair of small t-rails (c) mounted to the above sliders, that run perpendicular to the first set
- a pair of larger rails (d) that slide along this second set of t-rails, again with knobs that can lock their travel
- a router plate (e) mounted to these rails, tying the two together
- So far, then, the plate can move left/right and fwd/back unless locked down, and the workpiece rests on the 2'x4' surface
- straight edges to be mounted lengthwise (since exposed surfaces (b) move)

Note: small rails above fit perfectly inside the larger rails with no slack.
Several problems exist with this design, including:

- there is no mitering, only 0 or 90 degree routing (I have bigger problems!)
(ps: with all knobs loose, this rig could move diagonally in theory)
- there is no way to raise/lower the work piece, nor the router plate
- since the rails are all aluminum, stability/precision may be a problem

I can beef it up by sinking some of the rails into wood. And/or gluing or welding rails (b and c) together, but I'm not sure locking at 90 degrees is a great idea. But the biggest issue is that there's no way to adjust the height of the router or work piece.

Short of a set of chain-driven cranks, I haven't thought of a way to address the height/depth issue. Any thoughts welcome. Anyone tried anything like this? Thanks.......

Randy