How to construct a time capsule bottle?
It's really hard to assure that they won't be under load anytime in the next 50 years.  Obviously, the bottles only need to break once to be ruined.  Also, depending on the type of bottle, it might be hard to find lids that are waterproof, rustproof, and won't decompose.

I recommend using something sturdier, like large diameter PVC or ABS pipe.  It's easy to work with, you can cut it to whatever length you want, and it's easy to completely seal it off from the outside world.

You'll inevitably have at least some humidity from the air trapped inside, so I recommend adding some desiccant packets to help preserve the contents.