Pour just enough resin as to obtain this nice "bulge" over  the edge of the box.   Of course you’ll need to set the mold exactly level, otherwise resin will spill all over the place.

Now it is quite possible that you really had enough, so, set the mold aside for a couple of days, depending on the type of casting resin you used, and forget everything about it. More than anything else, don’t spend all your time wondering if you’ll ever see your precious master propeller again!  When the resin is properly set, the top of the mold –that nice bulge- can be finished flat by filing, grinding, planing, or whatever you like: I use a Stanley Surform rasp for this job. Remember that this will be the side the mold will lay on while you’ll be laminating the propellers, and this also will be one of the sides that will take the big squeeze once you close the mold; so it’d better be nice and flat. Finally, unscrew the base plate from the box and remove the packing from the mandrel, then a light tap should be enough to separate the half mold from the plate…. and you will be staring at this mess:

If you like chinese food, choose a restaurant that still gives you bamboo sticks, and not ugly plastic ones.
Ask then for a fork and a knife and pocket some sets of sticks.
 

Use those sticks with properly shaped points, to remove all the plasticine from the cavity of the mold: use only wooden tools so as not to damage the propeller. Don’t worry: all this will come out one day!

Take your time, and work with patience, DO NOT for any reason, move the propeller from its place: if this happens you’ll have to restart again from the beginning.
The propeller is held there (strangely enough) only by the release agent,
so avoid using solvents to remove the plasticine and go gently with those chinese sticks.

Cotton buds will also help in the final cleanup, until you obtain this result:

When you are satisfied of this first half of the mold, and you're quite sure that the propeller is stil solidly stuck inside the cavity, apply again your release agent, one or two coats.
Caution: the bottom spacer does NOT need the release agent: you will instead degrease it properly,
and slide it back onto the stem of the spindle, just before pouring the resin.

The top box can now be securely screwed onto the female mold, after a coat of release agent on the mating surface only. You didn't forget to predrill the holes and mark the position, right?
 

Here again, if you wish to save resin,       you can add some blocks       of scrap wood inside the box

Just in case, it is better to seal the outside of the mating surfaces with plasticine or/and a layer of masking tape.
 
 

By now you might get the feeling that this will probably end up a nice piece of work, therefore you will not feel happy if you have to bash it with a hammer in order to take it apart.  This is why, at this point, I place some simple extractors on the inside of the male mold.

Extractors are made with  8mm i.d. threaded bushes of the type used in making furniture, holding a flat point grub screw just long enough to protrude past the edge of the box.  Do not use conical pointed grub screws, they will damage the male half of the mold; grind them nice and flat. The bushing is held in place with cyano, while the screw must be well waxed, otherwise you'll leave it there forever!  Two of these units, placed at diagonally opposite corners of the male box, will generally suffice for a 6-7" prop.

 

Once again, try and calculate how much resin you need to fill the box, remember that K factor? And when ready, pour!

Obviously, you are dead sure that the release agent is all where it has to be,  that the mating surfaces are nice and flat, that everything is sealed,  and what had to be waxed  had really been waxed.....     .

Pour 'till you get again that bulge over the edge of the box: