If the garage has any windows, buy some frosting, like the stuff they use on bathroom windows. It comes as a plastic film and you apply and trim to fit. If the bad guys can see whats being used to secure the door, it makes it 100x easier to defeat it.
The emergency release is an easy way to get in. My wife and I are both tall enough to reach the release without needing a cord on it, so I just took it off.
A 1/2" long slide bolt on each side with a hole through the track will secure the door for cheap, but can only be opened from the inside.
If you need access from outside, a long throw keyed deadbolt could be installed on each side to do the same thing.
If its only a short term need, a pair of vise grips locked on the track above the wheels will work.
Long term you should unplug or disconnect the power to the opener.