How will concrete fill in?
This is a reasonable concern, but not necessarily a problem. It is not uncommon to have concrete forms the way you describe. The solution depends largely on the consistency of the concrete mix. A very wet mix will run all over the place, a very dry mix will hardly flow out the end at all. This property is known as the "slump" of concrete, measured in inches. Slump value range from 1" (very thick, hard to work with) to 6"+ (very liquid).

If you are mixing the concrete yourself, aim for somewhere in the middle. If the concrete is being delivered, talk to the supplier about your use case and they should help you.