It's ridiculously easy to break b64 because it's an encoding used not to generate data security, but to normalize strings for use in http/s URLs.
I propose a solution, which will be much more complicated, but which might make the links stay online longer: using AES encryption with a 256-bit symmetric key, a symmetric key generated through something unique that all topics possess. The output could be a b64 string, but it would only be correctly decoded with the symmetric key. Of course, there would be initial confusion, common to all experiences of adopting a new standard, but it would be very interesting when fully implemented by the community.
Well, this is just a suggestion; more different ideas, or even improvements to this model, would be interesting.