I had the same problems with the Bears model. I bought the tall mold and the stik fish mold and struggled to get a good shot when trying to inject with or without a tail in the mold. I had to modify the stik mold by adding an extra horizontal vent hole. I am not a machinest so the way I did it was by clamping 2 strips of 1/4 thick aluminum stock about 1/16 apart and used a variable speed dremel to run between the stock. Came out perfect. You don't need to run the groove deep. Just enough to have a gap from the surface of the top plate. Only need to put the vent on one half of the mold. Worked like a charm.
Interesting...I can only assume that you plan on using this to make a darker buried layer with a clear or lighter colored body. It's multilayer extrusion (Joe the Pliumber) and it's for adding a burried layer through a seperate channel. It injects one material inside of the other. Not to be confused with 2 port injection for laminates.