I bought a B+Q carcase and cut it down, machined the back rebate, etc.
The mistake was buying B+Q*, not the idea itself. If you have a tracksaw, or even just a long enough straightedge, it's easy. If it uses knock-down fittings, you might need to move the back ones. IIRC for me that was a 15mm flat bottomed hole (plunging router cutter from Wealden), with a corresponding hole edge drilled for the dowel. Awkward, but not impossible, and easy if you do the big hole first and make a jig for the dowel one.
My cupboard is full height for a bathroom. Anything smaller would be much easier because it would be much easier to handle the pieces.
E.
B+Q had three attempts at delivering the carcase undamaged. They didn't succeed. I was running out of time on the job, so kept the least worst one and got a partial refund. Honestly, I've almost never darkened their doors since -- can't face people with good intentions trying to defend an organisation with such broken processes.