Well I pulled the trigger and I got 2 evil 18s. A lot of my friends are telling me 4th order bandpass I'm not completely sold on doing it. One of my friends has the same Subs and paid for a custom design for his vehicle where sealed is 5.5 cubes ported is 14.8. SSA recommends 4.5 - 6.25 sealed. I realize that is strictly for a sealed box, but my question is that 5.5 that they have designed for him is only big enough for 1 sub. Is the design rules different with 4th orders?

 I'm leaning more towards just a regular ported but my thought process was for an extra 2-3cu tops why not have the same set up?

I would go with a ported box 4th order is usually hard to tune from what I have been told but don't quote me on that I'm no expert. Most 4th order boxes I have seen where for huge builds where they fill up the back of a Tahoe so a smaller 4th order may be easier. I know my friend who owns pj's car tunes said he did a computer model of his for 6 Memphis mojo's and when he built it the thing turned out to be tuned really high like a burp box. He ended up just doing a wall of slot ports. 

I have heard that too I did forget about it though. I thought about it all day yesterday and there are 4 local guys here running these this year. 3 of them are running 4th orders. So I've decided I'm going to go ported. 

 I'm thinking Centerport 15 cubes. @30z ... 228sq port area.

No I have not tested this through any program since I don't have a computer. This means I will probably start getting bashed anytime now. but if I'm completely wrong here someone could let me know it would be appreciated.

Yea you should go ported . I think the only good reason to do a 4th order bandpass is if you dont have the space to fit the subs you have and the port area you need in a flat ported wall

When using plexiglass or lexan or acrylic how thick does it have to be for just like a window 12-inch by 12-inch?

 Is this a bad idea considering it's a box with 2 evil 18 on two Crescendo 3500?

 Is one material better than another?