I have converted a bottle rack, but it is now just a seat, to make it a rack again would mean hacking off the welded on bars.
I had two square bits of stainless channel welded onto the sides at the same height as the centre channel. Cut a bit of ply and epoxied, bolts set uin place, foam and vinly cover added, then bolted into place.
I guess that instead of welding bits on down the side, you could fit an elongated U bracket (ie a flat bit of stainless bent to 90 degrees at each end at each end of the ply base and bolt this through the bottle rack at each end. Then the seat base would be removable easily enough.