I have done severla (if not many) large instalations.
Any of my onsite assemblies have been only for the reason that the frame will not otherwise fit into the building or room where the instalation will be.
Assuming it needs to be made on site, I see three options.
1) V-nail on site.
I have done it before. With an 810 none the less. So it's not an unheard of proposition. Compressor outside everthing else (all the other equipment)put into den. We just moved the Oscars off the shelves and put our tools there, and set up shop.
2) Clamp and glue (with L-Bracket).
Using a 4 Masterclamps, or 4 Ledsome clamps, glue up all 4 corners. While clamped, turn it over and place monster L brackets on the corners (in lieu of v-nails).
We used this method at City Hall. It was just after 9-11. The almost would not let us in the building with our equipment, and they really did not like the razors, and the nail guns. They had us run the moulding thru the x-ray machine, AND the metal detector. Of course the acrylic would not fit thru either machine, but they were even suspect of the sheet of acrylic.
Stanly clamps won't work because you can't get to the backside of the frame.
You can clamp and wait to dry, and then attach an L bracket too.
3) Biscuit and Clamp (with metal L brackets for added ooommpf). The biscuit or several biscuits will result in a strong and tight join.
With any of these opitons you may need to actually purchase a piece of needed equipment. Build it in the price, or just chalk it to equipment needed to run the business.
With any of these, you still need to hang it on the wall.
As The Goddess mentioned. Z-Bar, or it's cheaper cousin, The French Cleat will be easy to install and hang.
I have also done 4 sticks nailed to the wall (thru the compo), and assembled the frame literally on the wall.
I don't recomend this. The corners don't go together tighly, and you use a boat load of putty to fill, and it shows.
Walls are no always flat. This is a problem because unless it's Emafyl, or MDF, you can't bend the mldg to match the rise and recede of the wall. This can be very bad.
The other problem is that floors and ceilings are not always straight.
If you use the wronge reference point, or reference line, it may end up looking crooked on the wall. And once it's nailed in, it's really hard to move.
If it's assembled, and then hung, you can adjust the z-bar or cleat to make it look level to the room.