Low voltage long term solution

Good Morning,
So I have a couple of old security system siren boxes which are 6 Volts (powered by 4 x size D batteries)
I would like to re-use them to save them from landfill.
What I’m after is something “like” the ESP8266 \ ESP-Home setup in conjunction with a pair of relays to control the on \ off operation.
I don’t want to have to hardwire anything as this would be difficult due to the location.
Has anyone got any recommendations on a low voltage solution which isn’t going to break the bank or have me running to the shops for batterys every week?

Shelly Uni should do it. Two outputs that can control your relays. Tiny board, cheap, local control, directly supported in HA.