A picture of your stem & h-bar would help, this way we know for sure what you have and what to recommend. It sounds like you have an older h-bar with a 25.4mm clamp area and that the Wahoo out-front mount is designed for 31.8mm. As well a 1-1/8” Aheadset system stem that has 2 to 4 front plate bolts ?, that are typically 5mm ?

I’m certain Wahoo supplies the unit with rubber shims to accommodate narrow diameter bars, as I’ve used them and they hold fine

But if you don’t like the look of the out-front unit, you can get a mount that installs on top of the stem, under the Aheadset fixing bolt and cap. Just do a search on Amazon for Wahoo Stem Mount. I saw a few models.

https://www.amazon.com/K-Edge-Stem-M.../dp/B01G35DLQA

You can also use the Out-front mount and turn it 180 deg. so that the mount is on top of the stem. I do this on my mountain bikes. Or use the supplied Wahoo stem mount that can install on a bar or stem using zip-ties.