Holding it down does the job, but I find that I have to pack it in too tightly to be able to use the hold down method. If it isn't packed tight, then it floats up to the wax paper or screen that is holding it down.
I pour in layers, sometimes 8-10 layers. Mix enough resin and color (no MEPK yet) for the entire pour. Measure out just a little and add the MEPK, pour a layer 1/8 inch or so thick, sprinkle in the coffee beans, spice, pasta, rice, corn, whatever, give it a while to cure partially (I put it in front of a small heater), repeat, repeat, repeat until the molds are full.