When I make teas I don't use a bag. I just throw it all in and bubble it. I don't strain anything. I just pump it to my plants with a hose. I took out the pump filter so it's kinda like a tea and a top dressing in one.
I could see how a bag would be nice with some stuff though.
With what they charge for bubble bags/kits I'd think there's cheaper options. I've often see people saying to use paint strainers that go on a 5 gal bucket. That way you can just dump things in and bubble away like Padawan is saying but you can then filter the end product if you need to run it through something intolerant to solids (i.e. a drip/feed line).