There has to be an accessible point/connector in the electrical wiring of the aircraft where to plug the charger/maintainer without dismantling of the cowling.
This might find an elegant answer in the kind of trickle-chargers sold in car accessory shops, that plug into a lighter socket. There are commonly used in mobile homes, camping trailers and other vehicles with a 12V lead-acid battery, that are not used for several months, mostly during winter.
Make sure, though, to get one that fits YOUR battery - gel batteries might well have different requirements. And remember this is a trickle charger, its function is to defy trickle discharging, i.e. to keep a full battery full. It will and can not make a fully charged battery from a depleted one.
HTH,