@Yanng22: I've calculated Qts x Vas as 59.6 for the Kappa Pro 15LF so I would advise to build Cubo 15 Extended and not Extra Extended.
@ Steve k: If the angles between panels are not shown in the plan you can assume they have a 90 degree difference. The angle between the first horn panel and the baffle is 87 degrees, although if you use an expanding or filling glue type (like polyurethane or epoxy), you can get away with a 90 degree angle as the difference is less than one millimeter.
I want to build a cube15. I would like to use a 4ohm woofer and about 600w, as I already own the amplifier. which one could I use? with which cube 15? would the KAPPA PRO-15LFC go well with an extended cube? can you recommend anything else?
The KAPPA PRO-15LFC is a good choice for Cubo 15 Extended with about 600 W of amplifier power, if you use a 40 Hz high pass filter. To recommend any other driver I need to know your approximate budget and what brands are available to you/ your location.