I have seen a few. These two are the mostly recently updated and seem to be maintained:
https://github.com/luxifer/dokku-redis-plugin
https://github.com/ohardy/dokku-redis
The first one has 10 open issues, one of which seems to be potentially worrying from a security perspective (specifically this one https://github.com/luxifer/dokku-redis-plugin/issues/35 <dokku-redis exposed on 0.0.0.0 / public interface>).
Anyone here have experience and can point me towards the most stable one to use please?