Fit up to 3 of these badboys in your Series X.1, X or 4.1 (aka XL). 
Or fit up to 6 of these in a Series X.1 Plus!

Not only will it protect your beer from damaging light (amber tinting) it will also keep your beverages fresher for up to 3 times as long due to the Mono Barrier PET Polymer Matrix.
It also comes with a floating dip tube with cage filter. Is easy to undo and stack on top of each-other upside down with disconnects attached. 
The 20L Oxebar Amber PET Kegs come with a Green Pressure Relief valve rated to 65psi.

Dimensions:
640mm High
235mm Diameter