In commie times or early 90s in eastern Europe people (or at least my friend) used to have at home extremely simple device soda charger adapter with two PET bottle connectors on top and bottom, where on each end you could screw 1.5-2L PET bottle, I think one filled with syrup water, other empty, in the middle on side you screw small soda charger bomb (see link, think size you could put in small air gun, one small bomb cost around 13 cents nowadays, reasonable even for one time use 2L) and then you just release valve and I think you had to flip over bottles few times. It was incredibly simple and I'm amazed by pricey SodaStream popularity (even over soda siphon/seltzer bottle). Shame I can't find this adapter on mobile, so now think it had to be home made device which was very compact, no pipes required, everything within small adapter which fit between two bottles with bomb on side.
https://www.amazon.com/Soda-Siphon-Charger-Classic-Sodamaker...