Tons of altimeters out there, great prices, great reliability. You can't beat rocketry customer service, it's all people who want to help. Don't be afraid to ask questions from manufactures directly. I'm partial to PerfectFlite and Missile works myself.
Welcome to the forum. You won't know yourself once you use electronic deployment. A lot of the variability goes out of the deployment timing.
Lots of different ways of doing bays, have a look around and choose one that suits your skills and material choices. Birch(?) ply (I get mine from hobby shops) works pretty well if you are looking for something easy to work with.
Lots of different opinions, see what you think will work for you.
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.