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I built a box with a lid (38"L x 14"W x 6"H) that was going to hold a 5 gallon tank, 2 Air Zenith compressors, VU4 manifold, and a Accuair ECU. Completely wrapped the inside with Dynamat. I also had a center compartment that would hold some spare airline, cutters, PTC fittings, line taps, and a coiled air cable to inflate tires and run power tools if necessary.
What sort of space did that leave you? A buddy of mine has a bagged coupe and it looks to me like it would render the trunk useless for me.
This was in a 2012 Jetta which has a slightly (and I mean slightly) larger trunk space than the G37 sedan does. I don't have any pics handy but it was similar to this.
Below is a Jetta Hybrid trunk which contains the batteries for the hybrid system. My box was shorter in height and more shallow in depth (i.e. more space).
I actually have my compressors mounted on top of the tank using Accuair's Exo Mount to get even more space. First pic is the rough install with everything plumbed at the shop. Last pic is complete but at night. Doesn't show the space to well since it is dark but you see the idea from the top pic. I can take another shot during the day if you like.