Set you mold up in the normal fashion BUT, only mix enough Silicone to barely cover the model. Once that sets, mix 100% Silicone sealant with water (Walmart's Mainstay, $3 for a 10+ oz tube, sold in the paint section is MOST EXCELLENT) and fill the remainder of the box with that. It'll set in under 20 Minutes.The resulting mold will do everything one made with 100% Expensive RTV will at a fraction of the cost.
BTW, you can do the same with all RTV Silicones, even the High Heat ones like Dow Corning 3120. Anything that is not in contact with the model is filler and as long as the Sealant isn't in direct contact with the casting, it'll stand up to as many pours as the 3120 will!
FYI, you can determine how much Silicone is needed by estimating your needs using 20 gms of silicone per cu in.
Edited by Husky, 04 September 2009 - 11:06 AM.