Another thumbs up for b-rad.
Also had one of these that I used with moderate success. Although mine was for a normal saddle and not the brooks version.
your seatbag goes through the middle of it and a bottle on each side. Unfortunately I bought direct and the company did a bait and switch and it cost me over $90.00 with shipping and customs.
I used it on one 1650km trip. It worked ok??? But I have never used it since.