A_F_L_M, you're a total audio nerd. Good for you!

I can't make any direct recommendations. But having messed around with photovoltaics and inverters for a while, perhaps I can offer an off-the-wall (and off-the-rack) approach.

All those funky uninterruptable power supplies (UPSs) for computers provide super-clean, regulated, pure sine wave power. At their heart, they have a 12vdc lead-acid gel cell. Usually a ridiculously tiny one. With standard connectors. It would take nothing to connect this to a larger sealed lead acid cell. Or, for that matter, the 12VDC wet cell from a bulldozer (watch out, they often connect two in series to the starter).

The one wrinkle is that most won't fire up unless they detect a proper earth ground. That's up to you. In wet ground, a tent peg and a few metres of discarded speaker wire will do it.

Yes, it's a duct tape fix. I don't know if it fits the specific application you have in mind. But it may just open a new line of enquiry for you. Note that UPSs with dead internal batteries are just about free ...

Cheers,
Doug