Here's my completed (still need some aesthetic improvements but overall it fits and checks out) portable auxiliary battery. Relatively simple to put together. 50 usable Ah (LiFePO4), 20 A DC-DC charge capacity, 10 Amp MPPT built in. My DC-DC is outside the vehicle but the box can accommodate a Victron 18 or 30 A DC-DC unit.The battery has a max continuous current of of 60 amps.

2 Anderson SB50 connectors (one dedicated for unregulated solar) and 2 Powerpole connectors along with a USB type C, QC 3.0 port, one cigarette lighter port and a couple of 2.1A USB ports. The USB Type C/QC port has a small voltage read out but the box has a built in Victron Smart Shunt so I get capacity use data via bluetooth so didn't really need that feature (type-c ports are really hard to find). Choice of materials and components was in part driven by stuff that I had already purchased for a more standard dual battery installation (another G-34) which I chose to reuse for this instead of return. I had initially thought about building my aux box around a Trojan 100 Ah deep cycle AGM battery but that would have entailed a larger box, which would have weighed closer to 80 lb and would not have been practical or portable. My current setup has about the same usable battery capacity and weighs only 26.4 lb.

My next DIY project will be to do something similar to @Bumbo and build a DIY LiFePO4 battery from raw cells and BMS. I reckon I could fit about 100 usable Ah of capacity into the same form factor though safely securing the system into the box (the current battery fits snug and will survive off-road abuse) will be a challenge.