I've used platinum cure for one-sided molds; plastic masters and lego mold boxes no problem. I was always under the impression that they were a bit more resistant to mold expansion then the tin cure silicon. I've used TAP Plastics version:http://www.tapplastics.com/product/mold_making_materials/mold_making_supplies/tap_platinum_silicone/494
and more recently Smooth-On's version Mold Star:http://www.smooth-on.com/Silicone-Rubber-an/c2/index.html
. I use sulphur-free modelling clay for two-part molds if needed. Also I always wrap the lego mold box with masking tape to prevent seepage of the rubber. A small amount will always find it's way between the bricks (especially with vacuum or pressure) but at least the tape keeps it from flowing out.