You will needs some basic off the shelf parts to make a fire pit:
1) Regulator/hose 30 PSI (not a BBQ regulator) and a needle valve to adjust the flame size. I use brass 3/8 45deg flair fitting for the hose ends and the attachment at the fire pit.
2) Fire ring. This is the distribution system that is below the sand for your gas. If you have the ability to weld and make rings you can make your own using a 1/2 weldable (not cast iron) coupler in the center, 5/8 thin wall tubing and drilling 3/32 holes every 1 1/2". The ring should 1/2 - 1/3 the size of your container diameter.
Stainless steel will prolong the life of the ring especially during the winter when you forget to cover your pit in the first rain.
3) Fittings. A 1/2 NPT coupler (weldable), Brass1/2 to 3/8 45 deg flare (for attaching the hose to)(not used in this build), brass 1/2 NPT nipple (to attach the the ring to the coupler welded to the container)(not used in this build), brass1/2 NPT lock nut
4) BBQ sized propane tank.
I've put together an affordable kit of the basic plumbing supplies you'll need to build your own fire pit.
I offer several sizes of rings, stainless upgrades and fully auto systems for high end remote controlled fire pits.Fire Pit Kit linkUPDATE:
I now offer a new bulk head coupler that works in materials upto 1" thick. No welding required! see the last 2 photos on this page