I can design one for you from scratch to work very well with what you are after. I do charge for designs, but if you are interested, check out my links in my dog for more information.
The placement of the sub and port within a 90 degree angle is not very sensitive under certain conditions, which are usually met with most designs, but if you are looking into a custom one, it is proper to make sure that the output phase falls within that 90 degree placement for proper output. This can be accommodated if done correctly.
The reason of it being so popular is due to having the box inside the same cabin area as the listener. The phase coorellation changes from that being inside a trunk and if the design is constructed properly, the difference between placements can be great. You can essentially have the sub and port placement in any position, as long as the design is right.
It seems that the current popular Sub up port back is a way to fix broken details in the designs physical and acoustical performance. Physically, by being able to top mount the drivers for a possible better mounting security, and acoustically by auto loading the output of the port into a corner for low frequency extension. Which can cause either good or bad things to occur that may or may not be audible.
It also allows for the ability to hide noise and distortion.
Either way, don't rely on a general rule for placement. It can confuse the purpose of gaining a full response from natural characteristics of a design. If you are looking for lower extension, don't rely on corner loading in a vehicle......use a different setup because you may find yourself trying to overpower them to get it loud enough.
