You could assembly 3 of the 4 parts leaving out one of the 1 parts that forms the 3 rectangular slots and fill the mold with the liquid with a bit more and then press in the 4th insert and clamp it to pressure it and and expel the excess.
Would you be willing to share your files with me? I would be interested in making an injection type like the X1C I have. Of course I would publish it as a print profile under your original, on your page.