I just did the 2 NiMH mod too. 3V to charge 2.4V is too slow, esp since alkalines’ voltage sags a bit under load. Mine seems OK with the 6V pack I made up, you can have a guess at the charge time by feeling for when the 2nd battery gets hot. I get 10+ minutes from 60sec charge which is probly fairly conservative. You could just put a short where one of the AAs goes for a 4.5V charger if you’re worried about it.