I have been using the Dusty Strings Musician’s Stool for both my pedal harps, as well as all of my lever harps.
It is similar to the TAMA seat mentioned in the previous posts, but I have found that this particular muscian’s stool to be:
1. Light weight and portable
2. Useable (at least for me) for both pedal and lever harps
3. Able to “get close” to the harp without infringing on the boundaries of a pedal harp or staved-back of a celtic harp
4. Extremely comfortable for those very, very long gigs.