Ultimately, you can't. Space is really, really, really big and empty, though. Unimaginably so, which would make an incident be some really bad luck. In something like the lightsails, you'd just launch multiple simultaneously.
Space is so big and empty that even for something like the asteroid belt you could pass through any point randomly and on average be thousands of kilometers from the nearest rock. Perhaps even tens of thousands.
Is it though? To try and quantify risk I looked up how many metoeites hit the earth each day and its 18,000 to 84,000 that are over 10 grams.
Whiile the earth is obviously much bigger than a space craft, you would think should there be a bunch of craft flying around in the future, theres a fair odds of it happening every few years.
Things tend to collect around large gravity wells, that's just kinda how that works. I don't think you can compare the ISS in LEO to something traveling within or without the solar system.
It's extremely unlikely to get close to any object while randomly passing through its orbit.
Unfortunately, space becomes a lot smaller once you want to park yourself in a similar orbit to another object. Don't get me wrong, it's still massive, but since orbits trace out a path the problem becomes figuring out if two paths intercept rather than if two points intercept.
This is why access to 'space' will not really be impacted in the case of a Kessler cascade; you can go through low earth orbits on the way to somewhere else, but if you tried to stay in low earth orbit you'd almost certainly be hit by something.
Space is so big and empty that even for something like the asteroid belt you could pass through any point randomly and on average be thousands of kilometers from the nearest rock. Perhaps even tens of thousands.