Sure you could just strap your CO2 to your keg, but this allows you to drop a bottle in and out without unstrapping it each time.
Two parts, a ring for stability and a base for holding the bottle in place.
I may have overdone the thickness of the parts, I've not printed it yet, but the openscad file is there if you want to have a play.
I've allowed 63mm for the diameter of the sodastream bottle and 10cm thick for the body/ring. Let me know what infill settings you use if you print one!