They say to use 3/4 cup of rice flour for every 1 cup of wheat flour (so 3 Tbs rice flour instead of 4 Tbs wheat flour). Though the last time I made a roux I used 4 Tbs butter with 4 Tbs cup rice flour and it turned out okay. You might need to experiment; the ideal amount of rice flour may be in between.

Cool resource, thank you!