One dumbbell, a long walk, and a core that refuses to tip.
Brace hard and keep both shoulders even, do not let the load pull you sideways.
Carrying one dumbbell at your side loads you completely off-centre, so the traps hold the shoulder level, the forearms cling to the handle, and the core and back fight the whole walk to stop you folding toward the weight. That one-sided battle is the point, each side takes its turn resisting the lean the load is trying to force. It builds grip, posture and trunk strength that transfers to basically everything, including carrying the shopping in one trip.
One dumbbell instead of two, which makes it a suitcase-style carry. The two-handed version is balanced and lets you go heavier, the single-arm version turns every step into anti-lean core work, so the trunk and obliques earn as much as the grip.
Heavy enough that staying perfectly upright takes real effort, light enough that you never visibly tilt. Walk for the set time, swap hands, and match the work on both sides. If your shoulder droops or your stride goes crooked, drop the weight.
Let your grip be the limiter for a while, growing it is half the value of carries. Chalk is fair game, straps defeat part of the purpose here. If grip always quits absurdly early, shorten the walks and do more of them.
