Joe X

Any type of port will do fine as long as it's the right size, the best way to know what needs to be done is posting what vehicle you have and max dims for a box, also what amp you are going to use.

The box for a 18" is significantly larger, not all vehicles can fit such sub properly, if you can and depending on the sub you may and usually do get greater output for the same power.

Max dimensions for a box?

A very efficient ported box can be very efficient around tuning, a very efficient 6th order box can be efficient over a wider range of frequencies and you can control to some extent what those frequencies will be, but it's not like a bandpass can be twice as loud or 10 times as loud as a ported box just because it's a bandpass box. The simplest way to get loud is cone area, a single 12" for a blazer is a little thing to have, a single 15 won''t take a single extra watt and will be about 2-3 dB louder than the 12".

Straight up, you can put an elbow to each of the round ports so they don't get close to the subs below.

Well bandpass boxes are way less error tolerant than ported or sealed that be on the design or building phase so it's rather easy to get bad results, the other thing is that bandpass boxes will not provide meaningful output above the range for what they are designed for so usually 60-90Hz they have low output, this forces you to have a good mid-bass stage, with ported you are only limited by your LPF setting and that's way easier. Bandpass boxes can work if you don't mind the troubles along the way, for new users or if you are not needing the specific benefits they can offer I just wouldn't recommend them.

The 15" is the red plot on 3 net The 12" is the orange plot on 2 net These are compared on the same 900W power but the 15" can take 1100 RMS so it can even perform better. In this case I would go for the 15" as long as you can fit a proper box in your vehicle.

Greater cone area definitely translates in more loudness BUT the 15" sub will need a larger box, now one needs to know exactly what subs are you talking about and in which boxes to tell you what are the net differences between them. It's always a good idea to use larger size subs as long as you have the space for them.

If you have no deadening I'd say you can still do better. On the heating you may try setting your subsonic filter as high as you can that does not hit the performance you want, that is one thing to try, avoiding amplifying anything you don't need to.

This sub can work both in 4th and six bandpass boxes, the main advantage with bandpass boxes is more control over bandwidth , with ported high efficiency is mostly around tuning.

4 cubes is fine as recommended by sundown, my comment is regarding your port being thin and small, if you need to port up I would recommend 2 6" round ports for a well functioning port, sorry if the previous post was a bit too technical.

If you have clearance to the side you can port sideways, passenger side usually work, on your port, you need to be aware of 1) correct port area, 2)correct port area ratio, 3) keep the port inner end reasonably away from the subs. Your port area ratio is 30:1 and should be kept like 6:1, you have 30 square inches of port area total and sundown wants 16 square inches per driver or about 64 square inches total. Just a few things to bear in mind.

What type of music you listen?

Yes they do: https://www.skaraudio.com/pages/subwoofers I would recommend going with exactly what you heard if possible to avoid surprises, if not let me know which you choose and will run some software to compare with your current sub.

The 6" port would need to be 34" in length, since your box is 35" wide just running it along the width on the back to fire sideways would make it even not that visible so subs firing back port firing sideways, box spec would be 1.93 cubic feet @32Hz, you will need an elbow to turn the port, just use PVC pipe, as a temporary setup it seems ok specs wise.