Legos have been working great for me as well.
You will want to use the "pick a brick" function for their webstore so you can order the ones you need directly from lego that way you're not buying tons of sets to get random bricks.
http://shop.lego.com/pab/I use the 1x4 bricks up tthe 1x16 to make all my mold boxes.
Also one suggestion do not tape up the outside of the lego box as it'll trap excess air inside the parts which can cause bubbles along the interior wall of the legos if you depressurize it before the siliocn has set 100%. You can place tape over the bottom row of bricks just try not to tape over the crack between the brick layers.
You'll ocassionally get some silicon leak inside the brick ocassionally just pull it out with a sculpting tool and it'll be fine.