So if the battery holder is not the show stopper for you (@all)
The MEGA65 is for tinkering with, if you don't like the battery on board, design an adapter for a battery type/size you prefer more, document it and post it here. I might be interested
Not wanting to deal with the unusual battery type, this is what I did:
Bitte melde dich an, um diesen Anhang zu sehen.
Easy - peasy. Can be done with a simple pencil-tip soldering iron.
I got several CR2032 holders, but the bigger size makes it impossible to fit one directly on the pads of the original one. So I chose the holder that seemed most robust and mounted it with wires and a bit of double-sided tape.