If you're worried about your Eidolon Build being messed up, this can fit the #1 Eido build on this site as well with no problems.

"What do you mean this build doesn't have a single Umbral Mod??? "
"Why do you have [Intensify] there instead of Umbral??? "
"Who builds for Cast Speed?"

Personally, I'm not a huge fan of super meta builds, I make them so I can run content efficiently, but I like being able to have fun and moderately engaging gameplay when I play long missions. [Harrow] is one of my favorite frames and this was truthfully a build I made prior to owning Umbral mods, and could obviously be improved.
That being said.

What's this build about? (TL;DR)

• Speed Casting
  • Faster Penance Animation (Ability 2)
  • Faster Thurible Charge Time (Ability 3)
  • Faster Covenant (Ability 4)
• Staying alive maintaining your abilities (Sorry y'all, you don't get to just AFK Tank like some other individuals)

Why build the way it is?

• Speed casting. If the TL;DR didn't explain everything then let me tell you.
  - Aside from Condemn, when I first played [Harrow] I thought the inhibiting factors was casting animations, while Penance(2) is pretty negligible with faster casting being able to recast Condemn(1) as soon as possible means as little down time between your shields being down and being back at overshield functionally
  letting you be tankier, faster.
  -Thurible is a great ability purely just for the sake of letting you maintain overshield and Penance(2). The only downside is there's just a bunch of downtime trying to charge to full energy, I found [Natural Talent] and [Speed Drift] let me drain my max energy faster, giving me more time to actually fight enemies instead of just swinging incense in front of the enemy.
  -Ok ok, the "Faster Covenant" part probably doesn't make sense. What it boils down to is covering [Harrow] where he falls short.

1. Penance(2) eats Shields, making Harrow vulnerable
2. Thurible forces you to sit their and wave a lantern, making Harrow vulnerable -The best solution to dealing with his vulnerabilities is obviously, to make him invulnerable. Although you could make him be invulnerable longer, it's really just not fun to be stuck in animations. Building for cast speed on Harrow means he casts Covenant faster, giving him more time during the invulnerability to cast Penance/Thurible (either if not both).

I know this build isn't optimal even for what it's designed to do, but I wanted to share what I have.
IF YOU WANT TO OPTIMIZE THE BUILD:

• Replace "Redirection" with "Adaptation"
  -Why: Overshield is gauranteed to be +2400 and shields break fast and often, instead just be tankier during the overshield/invulnerability down time.

• Replace: "Intensify" with "Umbral Intensify"

• Why: I mean it's kind of obvious why. It's just better.

This build doesn't focus entirely around how much damage he can output, it just focuses on making him more fun to play beyond just a CC/CD buff machine.
Try the build. It's fun. And let me know what you think of it, rarely do I get to share my build.