Center for L810s

[FrederickP]

I use a pair of ADS L810s as the front speakers for my home theater system. I'm the original owner and have enjoyed them for 35+ years now. To my aging ears they sound fine.
I would like to add a center speaker to the system. I can buy it or build it. (I built a 15" amplified sub.) However, many posters state that the center speaker must match the timbre of the front speakers. Since I don't think purchasing a third L810 on the used market represents much of a solution, I am unsure how to go about figuring out what to buy. Any suggestions?
Budget is not a major problem; I want harmonious sound and doubt any $1500 center will mesh with my humble L810s. If I build, I'd frankly like to keep it as simple as possible. A plan for a single coaxial (12"?) would work better for me than an MTM. Finally, my 50" flat screen rests on a cabinet. The center speaker must sit on a shelf above the TV, so it should not be too bulky. Thanks for any help.