(it’s a 3xAA ‘beer can format’ light, maybe no longer available)
Old Lumens’ mod of that is worthy of consideration as a first draft toward what could be a BLF design.
A bit off topic, I’m more interested in 1xAA for my own use — but the 3xAA could be really nice as a gift for non-lithium-ion family and friends.
(being as I live in earthquake country, I try to keep handy a stockpile of lights and AA lithium primaries …)