G  - D - A - E        10/9 -  5/3  - 5/4 - 15/8
  \ / \ / \ / \         \   /    \ /    \ /    \
  Bb - F - C - G'        4/3 --- 1/1 -- 3/2  -- 9/8
    \ / \ / \ / \          \     / \     / \   /  \
     Db  Ab  Eb  Bb'         8/5 --- 6/5  - 9/5 -  27/20   

Here's a video about this Pentad tuning, made on Dec 26, 2013.
Just click the "play" control (ignore that loading... message if you see it; it will load then).

In August 2020, I silicone'ed the chord bars because the felt was too fuzzed up after some time. Here are some shots of that:

'Before': see how beat up, fuzzy and notched these felts are?

Here the chords arranged by 'key', G, Bb (or Gminor), and a bunch of 7ths.

Now they are sitting on wax paper, squooshed into toothpasty-lines of silicone.

The next morning: they are all dry.

Here's how gooey they are! They trim nicely with an art knife. You have to get those notches clear! This is much more tedious than cutting them in the first place.

'After': Back in the harp. Note: I put bamboo skewers in the chord bar slots to help get the felts closer to the strings. That way, you don't have to push so hard.

That said, the silicone was not a great addition: it was tougher than the underlying felt, which shredded(!), leaving silicone scabs hanging over various strings, damping them some or all the time. I've been replacing them as they go bad.

2019!
Here it is a few years later, and I've taken the idea of that diatonic pentad harp, with its "circle of minor thirds", or rather "Major and Minor of the same key", and cleaned it up a little.
I had a spare harp body with a cracked soundboard and a lot of bars made for 15-chord harps. I refelted them and left them uncut, and made this interesting video:



And then I cut three diatonic sets of 7 chords apiece. These chords are nominally triads, but use the "half felt" trick to add the fourth note in the top and middle octaves. You get tetrads unless you press the bar down harder. I also used foam mounting tape to get the chords closer to the strings, which makes it easier to play. So it's a lot cleaner sounding, and has some good transitions. But not much in the circle of fifths!

The chords are in the keys of D, F and Ab. Those who can multiply realize there are 21 chords there, but only space for 15. I swap parts of the sets out so I can hear what they are like. For a while, it was just F and Ab, like the Diatonic Pentad harp. Then it was D and F, which was great because I'm using the standard string schedule, and Ab basically has no bass that way. Another way is to take 4 chords from D, 7 from F and 4 from Ab. The chords I use are rich enough for substituting in most cases. It can also be organized as 5 chords x 3, IM, IIm, IIIm, V7, VIm for D, F and Ab, which is a little limiting, but can do some interesting things.

I arrange the chords in that same pattern as the pentad, roughly:

   
 GM7 A7  Bm7  C#o7     Bbm7  C7  Dm7  Eo7      DbM7  Eb7  Fm7  Go7
   DM7 Em7 F#m7          FM7   Gm7  Am7         AbM7  Bbm7  Cm7
  
   The VIIo7 chords can also be moved to the left of the IM7 
   (I have spare chord bar buttons in both top and bottom positions)
   
   Harmonically, the chords line up like this:
         
       DM --Em --F#m--GM --A7 --Bm --C#o    << D
      / \  /  \ /  \ /  \ /  \ /  \ / \
     Dm--Eo --FM --Gm --Am --BbM--C7 --Dm   << F
      \ /  \ /  \ /  \ /  \ /  \ /  \ /  
       Eb7--Fm --Go --AbM--Bbm--Cm --DbM    << Ab
      /  \ /  \ /  \ /  \ /  \ /  \ /  \  
    D#m--EM --F#7--G#m--A#7--BM --C#m--D#m  << B  key of B, no room for it