Hi FlyHigh,

I've had a quick scan of this thread and haven't seen it mentioned yet - bear in mind that the profile of box-section isn't 100% flat and might cause you a bit of grief mounting the rails...

Here's an exaggerated view of what I mean:

Click image for larger version. 

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Using the epoxy technique is one way to get around this - quite a few on here have employed that particular method - or getting your metal ground flat (a bit pricier...)

Good luck with the build - keep us posted with updates.

Wal.