I've made fake cum using stuff from MessySupplies; what worked fairly well for me was a mix of extra thick MSGunge (white) and VIVI stretch extreme (uncoloured).
Mix the MSGunge into 6-7 liters of warm water- this'll give a very thick opaque gunge. In a seperate bucket mix the first stage of the VIVI slime into 8 liters of warm water. You'll need a mixer and a sieve to get a really smooth mix. Pour the MSGunge into the VIVI and swish the two together- the MS should break into small suspended lumps. Finally add in the VIVI activator solution a little at a time. About 1/2-2/3 should be close for cum-like consistancy.
(Set the remaining activator aside for later use- using all of it turns the gunge into a single free-flowing lump, which is crazy fun to play in in it's own right).