Has anyone tried these battery packs ?

[BobVA]

If you use low self discharge NiMh they'll sit for months and still work OK. These are typically sold as "pre-charged" re-chargeables at the consumer level.

I haven't found them available as flat-top, but you can use button tops if you make your own "shell" out of heat shrink - I had a local battery shop make me up a 3 AA pack (welded connections) and heat shrink it, then I attached some DIY copper connectors. Don't try to solder directly to the batteries - it's dangerous and wrecks the cells.

Katie shared this idea awhile back and it works great: modify a 3xAAA cell holder to make appropriate contact with the battery connectors in the calculator, then load it with low self discharge AAA batteries.