I still kept them: those old, handy sized 2R10 battery-needing flashlights... batteries for these are hard to find, in Europe only Conrad still seems to sell them.
These Zinc-carbon batteries have very limited capacity, and are depleted before you know it...

But I found THE solution!! Dig them up, those lights, because you are going to use them again! All you need is a Keeppower protected 21700 with 5000mAh capacity. At least these are 75mm long, that's what we need... and don't forget to change the bulbs from 2,4V/2,5V to a 3,5V or 3,6V one!

Old meets new can result in some splendid solutions... this is one of them. Not only finally guilt free lumens to enjoy, there are MORE lumens from the same flashlight due to the higher voltage, and think about it... 5000mAh!! Even with a "heavy" bulb that draws 500mA you will have TEN HOURS runtime! And all that in such a tiny package! Too good to be true? No... in this case, it simply IS true :-)

A pity I cannot attach a picture still... in Germany they just understand how to offer the ability to attach pictures... at Taschenlampen-forum.de you can attach a picture JUST as easily as on Facebook... WHEN will that finally be possible HERE?? We live in 2020... !!