casemods

ok so basically I bought an amp and sub and box for $650 with tax from the local car audio shop. The prices were the same as cructchfield for the amp and sub. The problem is the "universal" ported box they gave me for $50. I did a lot of research and it turns out the box needs to be specifically built for the sub or it can damage it and not hit hard enough. The sub hits pretty hard as is but it definitely doesn't feel like it's getting it's potential My sub is a JL 12w3v3-4 and the amp is an RF P500-2 My main question is: Should I take the universal box back and get a $50 credit towards the custom box (which will probably be marked up a ton) or cut my losses, try to sell the box and have someone local make the box for me OR make the box myself (with the help of my dad who is super good with stuff like this) What would be a decent price to pay (before and after $50 sub box credit) from the car audio shop? From a person I find on CL or something? Side not: How can I measure the DB from the sub to judge not only by feeling but by actual numbers? here are some pictures of everything.