...OK so partially DIY. I got a pre-made PCB from ebay.
I used a big die-cast aluminium box. Used a centre punch, scribe and combination square (later realised the box itself was not square).
Anyway, it turned out well. Untested in the new form, yet -- the bare boards work well though.
I used a few bridge rectifiers with a dual-pole transformer, big electrolytic caps with a couple of ceramic filters. LM7906 and LM7806 for + and - 6V rails, which is too much voltage for consumer line level operation.
In a related note, the Topping TP60 amplifier I have was exhibiting quite a lot of RF interference though the speakers, presumably caused by USB traffic before and after the crossover. I added some ferrite beads in the signal path and the speaker output. It actually worked and the interference is much quieter now, although still there.
This amplifier, stupidly, references everything to mains earth -- the input signal, output and power supply. Worse, the signals and output go directly through the power supply. This flipping circuit board, Jen. Some chump has run the data lines right through the power supply. Amateur hour! I've got tears in my eyes!