I would pour a small footing with some #4's or 5's J-hooked sticking out the top vertically.
Then set your sono tube and pour it, essentially two seperate pours. Or, you could try and pour it monolithic.
The footing will add much needed support especially if you two are dunking
In all seriousness, it will certainly not go any where. I'd also tell you to buy the 3000 from the local concrete company if you can afford it. They are probably gonna hit you with a delivery charge because you don't order the minimum (8 yards), but at least you won't have to hand mix it.
Otherwise I would just use the bag concrete.