I've found the perfect sewing machine for my needs:
√ Portable
√ Heavy duty (sews thick leather)
√ Accepts thick threads
√ Walking foot
√ Cheap
One of these, variously known as a cobbler's machine, shoe patcher, boot patcher, and leather patcher:
It's a Chinese machine, loosely based on the Singer 29K. Reviews are generally very good, though it is recommended to file down the rough edges, give the machine a thorough clean, and and make a few modifications here and there.
I've had mine only for a short time, but long enough to see that the stitch quality is superb. And that walking foot, it rotates so that you can sew in
any direction!
The next adaptation I'll make is to raise the height of the needle bar so the machine will sew with industrial needles.
I'll post images of its capabilities ASAP.