I use magnum racking and find that it works pretty well (even better than it works for magnums, as the magnum racking I purchased allows for 3 mags, but then leaves unused space which MIGHT accomodate a spare 750ml bottle).
X racking Champagne bottles would be EXTREMELY dangerous. I find X racking to be almost too dangerous for “regular” bottles other than very standard Bordeaux-style bottles.
When I have overflow bottles, I just store them standing up around the base of my mag-racking section. There is some thought that standing up is the best way to store Champagne.
Wire bins with handles to restrain the bottles that don’t stack or fit in racks. They are on their sides. Not beautiful but works well. Krug wood boxes also work well.