When I make my mould boxes, I use Lego bricks. But there's a big problem you have to fix before you can. When the bricks are snapped together they "look" nice and tight ----- NOT! Used as they are, the silicone in it's fluid state will find any gap anywhere and the Lego boxes leak like a sieve.
To solve that I started using modeling clay by pushing it into the gaps, it's a lot of work not just applying it, but cleaning up afterwards too. Very messy ----
But I've found something else to use which works fantastically, Plumber's Teflon Tape. I just lay strips of it on top of one brick layer before I snap on the next level. It seals the joint and then afterwards, being Teflon, just peels right off.
Another plumbers item I use it the glue, Lego is made from ABS plastic, so is the black plastic pipe plumbers use. I've made a number of sides up by gluing the bricks together, saves on using tape all the time ---