I have another look at profile rails. Is it possible to machine the standard hardened rail? I'm thinking if I drill holes into the rail at say 100mm intervals and use them to support it then that should be pretty much as good as the proper stuff? Certainly a lot better than nothing.
What about using one or two of these:
http://uk.rs-online.com/web/search/s...duct&R=4485512
Perhaps I could use two mounted perpendicular to each other to better take care of the forces in both directions.

Those numbers I got with your spreadsheet are a bit of a bodge. I just entered 1200mm for the width (to simulatie the I section bit) and put a low value for the supports - I figured that'd be near enough reality! I was more trying to compare the different cross sections than get absolute figures for deflection. I think I'll modify the spreadsheet to better fit my needs. By the way thanks for making it!

I've tried using the spreadsheed for the 20mm rails aswell and figured that they would be fine.