For dual deployment, you need a DD altimeter, two charge wells, a wiring kit, a battery and battery holder, risers to elevate your altimeter at least 1/16" above the sled, a sled, one or two pieces of threaded rod, two forged or welded eyebolts or stainless U-bolts, two terminal blocks, and a switch.
For redundant systems, double everything except the eyebolts and threaded rods. Of course, I did not include locking nuts, regular hex nuts, and wing nuts, and washers for all.
You'll also probably want shear pins, and a way to bond the top of the coupler to the payload bay, like with thumb rivets or metal screws and weld nuts.