For casting purposes, I use LEGO to make my mold frames. They are easy to build, they can recreate a mold frame perfectly and can be re used over and over.
NOTE***PLASTIC BREAKS DOWN AT 170 DEGREES F. STEAM IS CREATED AT 212 F. "DO NOT" PUT YOUR LEGO THROUGH THE DISHWASHER. IT WILL WARP.
Seriously. Some people have done this to clean their LEGO.
Mega Blocks...yeah. Mega Blocks are cheaper k/o versions of LEGO. I do not use them. In fact I have thrown most of mine in the garbage. I "DO NOT" recommend using them. They react with the mold release and the silicone some compounds. The Mega Blocks plastic breaks down and becomes sticky. They lose their color as well. Not to mention they don't hold together even remotely as tight as LEGO.