Large pots sometimes come with a small steaming basket and a separate pasta insert. The steamer basket works great for vegetables and small items, but what do you do when you need to steam big, cumbersome things like lobsters or Dungeness crabs, Philly Ray wonders on Chowhound.
You could use the pasta insert, but it's probably so deep that there isn't much room for water underneath. True, you don't need a huge volume of water for steaming (tim irvine steams broccoli just fine in the pasta insert over just 3/4 of an inch of boiling water). But you'll probably want to look for a medium-deep steamer basket that doesn't sit as deep as a pasta insert, grampart says. Just make sure the lid fits, creating a relatively sealed environment to keep the steam in, jljohn notes.
Another solution is to layer a food-compatible object under your crabs or lobsters to raise them above the boiling water, something that won't act as a barrier to block the steam from rising around them, jljohn says. You could layer clean, golf-ball-sized rocks at the bottom of your pot. Or use a cake rack, even a rope of aluminum foil formed into a figure eight.