I've seen that kit before. Basically a Chinese copy of a Fender Blues Jr with a couple of basic modifications.

But that head version really doesn't lend itself to mounting in that style of combo cabinet. You'd normally have the amp chassis held on by a couple of bolts from the top and a couple at the lower end supporting the chassis from the sides. There are no means of doing that with this amp head design.

The best you could do is screw the chassis to a backplate which then screws to the back of the amp. But that means all the valves and transformers will be pointing into the amp, so any valve removal or checking will be impossible without taking the chassis out completely. You may also run into problems with the amp components then actually touching the speaker chassis so it may simply not fit! The chassis itself is 36cm long, which I'm guessing is probably a good 6 cm shorter than the length of the amp chassis in the Custom, so you'll also have gaps at the ends that will need plugging.

And I'm afraid Doc hasn't really thought this one through with regards ventilation as unless you create some grilles in the top of the amp, the valves and transformers will be sitting in an enclosed section with no through-ventilation at all. You can't make ventilation slots in any backplate a) because of the high voltages present and b) because you've still got the metal base of the chassis in the way.

The one benefit of the (probably) shorter chassis than the Kustom is that it would allow you to create vents at either end of the chassis.

Note that you'd want/need to create some panels for the open ends of the chassis, ideally of metal, that are grounded and help cut down noise pickup by, and emissions from, the amp. The wooden back panel would also benefit from a thin metal sheet or shielding tape, on the inside, for the same reason.