I don't even know what one is.
OK Google educated me. I saw some on the internet at prices from $800 and up. Wow! I think I would just rent one if the church didn't have one for wedding use.
If not, I would try to find one (quick) and take measurements to build another.
Are you meaning a bench that the bride and groom kneel on? Or for them to sit on at the reception? Either way, it sounds like a beautiful thing to make, and a wonderful heirloom for them to keep. (Or, am I totally wrong about both guesses?)
I think that's a lovely idea, their own kneeler. A totally non-wood suggestion, but perhaps you'd like to use fabric that will be one of her wedding colours?