Johan, an option that I have used successfully in the past when I’ve lived in places without easy access to proper hills, is a long-ish tire drag while poling. I’ve done this using a waist harness (climbing harness), to attach to the car tire being dragged, and with a chest harness. I find that the chest harness also really engages the trunk musculature so there’s an added benefit there. The trick is finding a tire of the best weight to allow for a workout of the length you’re looking for. I have several tires ranging from 15kilos to about 8kilos.
Good luck!
lyle