This looks like an interesting idea - I'd be interested to hear feedback from anyone who has tried this, as my current loft aerial (just a "freebie" dipole, mounted at the top of the rafters) is only just acceptable with my better tuners. Something like this could be made to fit fairly easily up there - much more so than a traditional multi-element type.
I'd be tempted to join the aluminium sheets with M3 nuts and bolts, with shakeproof washers to help improve the contact. For the connection to the downlead, the same nuts and bolts could be used to attach some solder tags. The co-ax could be soldered directly to these, or short wires could lead to an intermediate connector, like a terminal block or a socket of some form. Easy to build and take apart
