V40H here:
https://www.ebay.com.au/itm/VARTA-1-2v- ... SwsB9WA~MQ
V80H here:
https://www.ebay.com.au/itm/2x-Varta-V8 ... SwvApaIf6M
V80s will give better run-time, but are a tight fit. You may have to do some grinding.
V40s will be a lot easier.
You don't need to pull the module out of the top so the solar panel shouldn't be exposed. Too late now I guess.
Is it a MkI ?
Does it run off an external 3V source ? Would be wise to check this before investing in batteries.
Rgds,
Andrew.