Timely question as this is exactly what I'm about to do. As far as I know it will work fine. It should avoid any kind of spike to the alternator when the BMS/and or charger cuts off the charging to the house bank, as the start batteries will absorb the momentary excess charge that was going to...
Here's a Rich Solar 160 watt cigs panel for $509. Doesn't really fit your size requirment though. I think thee is a company called Merlin Solar that might make a custom flat panel.
https://richsolar.com/products/160-watt-cigs-flexible-solar-panel