Nice! Killian does some good work, Im really happy with the X-LS cabinets I got from him, & if I had the room for a center channel, I'd definitely build one. Even if i used it as a sound bar for my TV, its would be worlds better than the built in speakers...
But yes, the x-over is meant to fit between the two center braces.
To reduce interference with the spundwaves of either internal woofer.
I hope you enjoy your kits as much as i have!