You're saying vertically behind the plastics on the sides of the cargo area? There is definitely room aft/below the jack as well as on the opposing side; although there isn't much airflow back there, so for something that gets hot, I think it would nullify any gains of vertical mounting. Also hopefully you grabbed an Orion XS 50A not the older Orion-Tr 12/12 30A as the latter runs much hotter.
For anecdotal reference, I have a trio of Orion-Tr 12/12 30A chargers in our Sprinter, mounted vertically with 3 40mm fans at the bottom of each charger blowing air across the heatsinks (blowing it upwards across the heatsinks which dump into the electrical compartment, which is then ventilated by 2x 200mm intake/outlet fans for constant airflow). After about 30-45 minutes of charging, they start to derate slightly, after 1.5-2hrs of charging they are notably derated.
In the Grenadier I have a Orion XS, mounted horizontally under the rear seat (mounted fairly low towards the rear of the compartment) with a single 40mm fan blowing air at the bottom side (perpendicular to the charger) for simplicity. That unit will happily charge at without derating for up to the 1.5-2hr mark (full battery)