Really don't even need a device. I practice balancing on one leg with my eyes closed while brushing my teeth every morning (I've progressively made it harder over time). I mostly do this to keep up my balance skills for mountain biking and it's helped quite a bit, especially during winter when I ride less.
You mentioned progressively making it harder, I’ve seen the following strategies to make balancing more difficult. Combining them all can be challenging:
1. Arms crossed on your chest
2. Eyes closed
3. Swinging your head left and right, like an exaggerated “no”
on the opposite end of simple-or-cheap, e-foiling Does train your balance. esp. in windy "cabbage-salad" waves with medusas and rubbish floating around :/