Hot damn, first time actually posting here.
I make rootbeer/cream soda/other pop for my roommates (not big beer guys like myself), and I usually just force carb to bottle the stuff so I have kegs open for beer. I've had some great luck using a makeshift counter-pressure filler.
Us a picnic tap with a number 2 drilled bung (I think? whichever one fits bottles) and put some racking tubing in on the other end. It the bung should fit into the bottle snugly, where the tubing reaches the bottom of the bottle and the picnic tap is on the outside.
Set the pressure on the keg to 3-5 psi and start to fill the bottles. Make sure the bung is air tight. When the flow stops, "burp" the bung to let some pressure out. The counter-pressure keeps all the CO2 from foaming out of the pop and leaves a proper amount of carbonation. Hope this helps!