Hello Neil,
The Mirror box actually has 4mm clearance on the sides and against the front, once painted there will be 4mm felt pads glued as gliders to stop excess movement.
Yes, M8 'T' nuts recessed so they end up flush (actually 0.5mm below the surface) you can use either zinc plated or stainless steel ones. The secondary struts have threaded inserts. I got mine from Peter at SDM but they are available from places like Specialty Fasteners. Lookie here:
https://www.essentracomponents.com.au and search for ' spring threaded insert '
The truss poles will have a plate joining each pair of poles and a bicycle seat post quick release clamp as the fastener.
The wheelie handles will be attached to the rocker box with M10 T nuts similarly recessed into the plywood.