Hi Andy,

With regard to mounting the supported rail onto Ali profile, this was something I was pondering over a while back in case I upgrade to supported rails. I have pretty wide Ali sections with double slots, but they do not line up with the bolt spacing. I had decided that I'd probably line up one set of holes over one slot, sliding a long steel tapped nut plate into the slot to bolt into, as Ross has suggested. For the other side, I'd just drill and tap direct into the Ali. My profile has quite a thick wall section, so I think I'd get away with it, but the sort of part you might end up buying might be a bit thin. Some people use the 1 - 1.5xD guide for depth of thread vs diameter of bolt.

Another option would be to bolt a plate (say 6mm thick) to the single slot profile, using countersunk bolts into a nut plate inside the slot. The plate would be wider than the profile, and be wide enough to bolt the rail onto. Less stiff than some other options, but a possibility.

Or maybe you'll get lucky with an off the shelf double slot part.