Reaching out for some help here. I am building an cabinet based.on the SWR Triad and had a few questions. Any input would be greatly appreciated!!!

1). How big should I make the vent at the bottom of the cab? Or if anyone out there has this cabinet can you tell me the measurements of the vent. I have read that as long as you stick to the specs and dimensions of an original everything should be okay. And I have to a 1/16 of an inch.

2). I would like to put a crossover in the cab itself so all highs go to the horn, mids top the 10 and less to the 15. What kind of cross over would I need and where can I get one? Or is this something I can build?

3). I am using speakers that I have laying around to make this until I get some new ones. but for now the 10 and horn are both coming from a GK Neo 410 and the 15 from my buddy which is 300w at 8 ohms. Will this speaker combo work and what is the overall impedance. I can not find anything on the GK spec wise. Anyone know?
In terms of a crossover, you could buy a generic, off the shelf, but it won't be designed with your speakers in mind. Conversely, you could make your own crossover, but it'll require A LOT of research and designing. Phil Starr would be able to give more of an idea on that, I've only got experience with first order crossovers, which are simple, but require very high quality drivers to work properly.
You need to be designing the cabinet for the speakers. Just putting any old speakers into a cabinet will have unpredictable, and often bad, results.

WinISD is the program you want to use to see how a speaker will interact with the box. Alternatively (I can't remember if WinISD allows you to calculate non-round ports), try this out - http://www.mobileinformationlabs.com/HowTo-1Woofer-Box-CAL%20Port%20lenth%201.htm.

With regards to the crossover, you need to use high quality inductors, capacitors and resistors, and it takes a lot of designing.

From looking at an SWR Triad, it seems that the idea was to create a three way cabinet? If I were you, I would look at constructing a fEARful cabinet. You can get the parts for the crossover, the plans or even the cab as a flat pack, and buy the perfect drivers for the cabinet to get a result that is planned and sounds great!
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Thanks so much for the hot tip guys! Nutter101, I found a plan on that site pretty much the exact type of cab I am looking for, the 15/6 tube. Do you guys know of any secret squirrel sites were I can get the speakers a little cheaper. I don't mind paying 200 for the 15 on amazon but if I can get it for less somewhere else that would be great.