Size is key. Extending the height of my box behind the passenger seat worked well. This is the details of the battery I have:

The Renogy Smart Lithium Iron Phosphate Battery enables auto-balance among parallel-connections and provides more flexibility for battery connection. The integrated smart battery management system (BMS) not only protects this 12V 100Ah LiFePO4 battery from various abnormal conditions but monitors and manages the charging/discharging process. The state-of-the-art battery cells ensure a long cycle life and exceptional discharge performance. If you’re searching for a deep cycle battery for caravan, cabin, or marine use, then this lightweight, auto-balanced, ultra-safe, long-cycle-life lithium-ion battery is the perfect plug-and-play solution for you!

The Renogy Adventurer Li and Rover charge controller are fully compatible with this battery.

Bought for £509 at https://tenergise.co.uk/product/12v-100ah-smart-lithium-iron-phosphate-battery/

Monitoring Screen for Smart Lithium Battery Series

The Renogy Monitoring Screen for Smart Lithium Battery Series is a high precision meter designed for Smart Lithium Iron Phosphate Batteries in off-grid energy storage systems. Instead of measuring the current flowing in/out of the battery bank using a shunt, it can communicate directly with the battery management system (BMS) and obtain more accurate state of charge (SoC) readings compared to traditional battery monitors. Other battery bank information including voltage, current, capacity, remaining time, warning code, and number of paralleled batteries are also available on the monitoring screen to help users avoid abnormal conditions and extend the lifetime of battery bank.

Plug and Play

• Simply connects the monitoring screen to the battery bank using an RJ45 communication cable for real-time monitoring.

Accurate Readings

• Obtains battery bank status directly from the battery management system for precise tracking and prediction.

Comprehensive Protection

• Displays straightforward warning codes for the quick recognition of potential abnormal conditions and improper operation.

Easy Operation

• Shows detailed battery bank information at the push of a button without the need of system configuration and calibration.

You can buy this for anywhere between £10 – £30 depending on offers. I’ve bought a couple (for van and for in-house charging) once from Amazon and once from Renogy directly. https://uk.renogy.com/monitoring-screen-for-smart-lithium-battery-series/