Borgelt B40 questions

I am planning to replace the mechanical backup vario in my glider with an old B40 I have bought. That should avoid interference with my main vario which is on the same TE line, since both would be pressure-transducer based.

The B40 I have has a very tiny speaker mounted on it, not much more than an inch in diameter. Would a larger more conventional speaker with a paper cone that is 2 inches or more in diameter work better? I.e., have a louder and more pleasant sound?

The B40 can have a 9V battery mounted on its rear for backup power, removing my one objection to having an electronic vario as the backup. But what type battery would be best? A lithium battery (non-rechargeable) would have the longest life in this role (probably never used, but really want it to work for hours if ever used), and may be more immune to the extreme temperatures the glider cockpit is subjected to, both in the air and on the ground.. OTOH, is there a danger of a such a lithium battery bursting into flames? Yes we carry many other small lithium batteries in the cockpit, in phones and PLBs etc. But those don't live in the cockpit between flights. It gets pretty hot in the trailer on summer days.