So I had some time today and got the measurements for you. I used a piece of cardboard to make a template then measured it:
So this is my design:
Its basically the same design except larger to cover all the inlets and also three triangle shape cutouts so the basket will "ride" the three blades, this will allow the blades to act a valve. I suggest using a grill instead of a sheet for the bottom but if you must use a sheet then use large holes. For the lid, I have decided I want to use a inexpensive clay saucer that will just sit on top of the basket.
Normal view without clay saucer lid:
Transparent view without clay saucer lid: