Thank God that Fender makes crappy amps!
Turning to the physical construction problem,
I have solved this by the simple expedient of buying a piece of crap.
The Fender Pro 185 is admittedly a loud amp.
Unfortunately, its 'sound' is crudely synthesized by transistor/op-amp technology.
Typically, these amps (like all amps) develop problems like crunchy,
noisey volume/tone pots, and crackly cutting-out symptoms, which are
notoriously difficult to hunt down and/or permanently fix.
So, such amps have the double-strike that they are both NOT TUBE AMPS, and they have as many or more problems as tube amps.
As a result, their resale value is similar to that of an old foreign car, which offers expensive repair bills and little else.
Yet, for the savvy amp-builder, these boxes have one redeeming feature:
They are about the same in construction as a good Fender Twin Reverb (Tube version).
Obviously, the speakers, having to handle the same power, are the same,
and so is the cabinet. To buy these parts individually would be
cost-prohibitive, and to build a cabinet, while a worthwhile project on
its own for a woodworker, is as expensive as just cannibalizing one.
So I picked up such a crappy amp used, for under $200.
Speaker and Cabinet problem solved:
These speakers are just about the best I could get,
because they are even better than typical Marshalls.
They are designed for abuse, and don't require mounting
in a sealed cabinet, but can handle pretty severe excursions,
and a lot of wattage.
The equivalent 140 watt Fender cabinet without amp would run about $500 new!
But the additional bonus is of course a virtually clean chassis,
with all the mounting parts and re-useable hardware,
e.g., A.C. cable, On switch, silkscreened frontplate etc. etc.
This is another good reason not to sneer at crappy old amps,
that would cost an arm and a leg to fix:
They are natural part-sources.
I may have to cut back or rebend/fold the chassis,
but that is a small price to pay for securing a solid,
practical platform for my amp project.
This was a case of money well spent on this project.