For larger drills (36V) a Pelicase or equivalent is probably best.
For a smaller drill (18V) we pack it in a kids large, insulated lunch bag, with carrymat glued inside for extra padding. There's usually room for bits, bolts etc. inside too. That goes inside a dry bag, then inside another tough, disposible bag (a rubble sack), which is there to keep grit away from the drybag. The whole lot goes inside a tackle bag, lined with more carrymat. This only takes up about half the tackle bag. You could use two drybags if going somewhere very wet. Care is still needed as the drill isn't in a rigid container.