With your limited space I would port around 50hz. That way the ports will be short. Two 4"/100mm x18"/46cm long ports will give you 55hz tune in a 30liter cabinet. If you try and tune lower you need bigger ports. And they will not fit in the cabinet.
558x558x508 for the Jack 15
457x457x558 for the Jack 12
381x406x508 for the Jack 10.
All of these designs can play pretty loud. I dont know what type of volume you need. You could also power these Jack's with as little as a 10watt amp also. 10 Watt amp could run off a couple of batteries if you want to be extremely portable. Or you could use a small battery for more wattage/running time.
Sort of just throwing ideas at you because I dont know what exactly your trying to do.
The plans are purchased from Bill's site for 15usd each. If you order more plans they become cheaper.
The green and the red bike on this picture sounds really good. I don't know how low it goes.
I don't know if this gives you any clue at all. I haven't got any specs, the only thing I know is that it is NEO units, and mine is not.
But as you see they managed to fit two 15" as reflex.. But I think I figured that it is only possible with one of my PD.154.. am I right?
Some of those are very tall. Very tall. If this is just a matter of sounding good then a single Jack 15 would be all you need. Have a external mixer with EQ and your set. You will need to power all this with something also.
The reason I said your 15's would not fit is because you said it had to fit in the cart. But if you are not limited vertically then anything would fit. BUT a single Jack 15 would probably be just as loud as anything there and use less watts and sound better. Bill, the designer, has done an awesome job on the design of the Jacks. Very efficient and can get pretty dang loud.
You need to try and keep this under 25m/s or lower depending on port size. WinISD just ports at Fs or close to it. SO you can tune lower if you wanted to. BUT for outside I would tune at tuning or higher to gain some spl.