I have some runners that were originally in some sort of server racking thing. They are double extending, and go from about 400mm to 630mm. Rack mounted servers are often fairly hefty things, so I should think they would be OK weight wise for you.
There are 11 of these runners stashed in a box under my desk at work, and I had intended to make summat like you describe for the garage, but I doubt I'll ever get 'round to it. You're welcome to them, but unless you happen to be visiting Kent one day, I think the postage may be dear!
I couldn't see the wood form the trees, I've had a chat with one of the guys and he reckons there's a pile in the warehouse I can have thanks for the thought
