Mike,

I have always thought that maybe if I had left them alone until it was fully cured I'd have been fine

You might like to consider a two-part silicone (http://www.tapplastics.com/product/mold_making_materials/mold_making_supplies/tap_platinum_silicone/494). Through-thickness cure is faster (around 2 hours) since you aren't relying on air/moisture to diffuse into thick sections of one-part formulations. The Tap stuff is low viscosity, but they carry a thixotropic additive that can be used to get a gel/paste consistency.

Sam