How best to fill the bottom of a PVC Tee with resin, plaster or some other material?
Operating with a two stage fill is a good start. Consider also that the dead space at the bottom need not be anything special, generally speaking. Cut cardboard, stuffed newspaper, just about anything at hand. Create your 1 and 2 shapes all in one step or in pieces, as you like.

Once in place, seal the top surfaces of 1 and 2 with a flat component, perhaps another piece of custom fit cardboard or two.

This now forms the desired surface onto which you can pour food safe resin available at craft stores and hobby shops and online resources.

The last layers are to reduce the amount of leakage into the mass below as well as to reduce the amount required. Even some modeling clay to seal the edges on the inner pipe may be required.

The food safe resin provides for safe pellet access. You can pour in multiple passes and angle changes to get the thickness you require.