As a latex thickener, there are actually a couple of products out there. One is called "thixotropic". It is used with both silicone AND latex. With latex you don't need more than a drop or two per ounce to get the latex to thicken up more. Monstermakers and FX Warehouse are likely to have thickening agents as well.
Alternatively, you could also mix in a bit of talcum(baby) powder to the give the latex a little more body or other non-toxic powders. I've also used flour in emergency situations.
I'm sure there are other methods that work just as well or better.
~DD