There is a thread on this forum called “weak horn?” That covers many options. I posted on it identifying the proper adapters to put a new horn on (I believe they are Civic part, but they work just fine for the clarity).Of course there are probably other options but these are known to work.
Someone on the Clarity PHEV Facebook group just posted a horn replacement video about a week or so ago. Not sure if the video is available anywhere else though, but he went through all the steps to do it.
I used to drive an Acura RDX myself, and I have to agree. It had a pretty impressive horn. So, I can understand why you'd miss that sound after switching to the Clarity.
I've been considering replacing my Clarity's horn, too, but I haven't taken the plunge just yet. It's one of those things that's been on my mind, especially when I find myself in situations where I wish my horn had a bit more oomph. If you're still on the lookout for options, I recently stumbled upon some interesting horn solutions at https://bosshorn.com. They offer some unique alternatives that might just be what you're looking for.
The hardest part was removing the plastic cover at the front once you open the hood. But once that was out... removing the old horns, installing the new adapter, and remounting the new horns was pretty simple. Took approx. 30-45 mins but I was going extra slowly to make sure I didn't break any plastic tabs when removing the plastic cover.
I found this video on YouTube to be quite helpful.