A 10-seconds digital reverb with very few memory (64KB)
Take a look at this https://ccrma.stanford.edu/~jos/pasp/Freeverb.html or https://github.com/highfidelity/gverb. Gverb is clearly the better sounding one but harder to implement.

Not sure if you can squeeze it into 64kB but it's worth a shot. The length of the delay lines is scalable to some extent. It roughly corresponds to room size with small delays being small room. 10s in a small room sounds more like bathroom and less like a concert hall or a cathedral. For really high reverb times I would recommend a floating point implementation to avoid instability and noise in the feedback loops.