For a cheap effective valve I used a sprinkler valve
I believe they run on 24v. I used 3@9v batteries wired in series to get 27v. I fired the cannon with just a simple push button. You could easily wire that to your carputer and fire it from there. My cannon was made from schedule 40 PVC (white) DO NOT USE SCHEDULE 20.
For a 12v compressor look at two different options. A 12v compressor for air horns. There are commercial version which would work great. OR look at converting a air conditioning pump. When I was into 4X4'ing this was quite a popular thing to do.