> These things will fly at high altitudes.

That seems improbable? Lithuania is only a couple hundred miles across and these balloons are completely at the mercy of wind.

I would expect the balloon owners want a quick hop over border security. Chasing them down over hundreds of miles of potentially private property sounds like more trouble than it would be worth.

Helium or hydrogen balloons are easier to fly at higb altitudes, but you might be right that shorter hops would be more controllable.

What is shorter hops in this sense to you? If you make some sort of weather balloon where the balloon never bursts, yes, they will travel very far. If you use a cheaper balloon that does burst, they don't travel down range very far in comparison. As I posted previously, depending on the balloon/parachute/payload, our little "experiment" went just over 100+ miles. That seems fairly acceptable. You could start 50 miles away from the border, and the recovery team would also be 50 miles away from the border. That's a pretty good buffer from any security teams working border crossings.

Shorter hops as in tens of kilometers rather than thousands.

I think you can just use mylar, but maybe you'd need a net over it?