How to make the cheapest workbench somewhat workable

Now, while there are many of you who can afford either singly or by two or three clubbing together to fix up a shop in first-rate style, there are also many who cannot afford even so cheap a bench as that just described. What can you do in such a case? Only one thing—patch up a bench out of whatever old stuff you can find. Patched-up makeshifts are not to be recommended, except in case of necessity, but when it comes to the pinch, and a matter of having a bench made of whatever old materials you can find or having no bench at all, by all means make one of boxes[78] and anything that can be worked in. For of course the boats, skis, squirrel-houses, and so on, must be made!

But, whatever you patch up, make it solid and strong. Do not try to work at a rickety, shackly apology for a bench that shakes and jumps and sidles all over the room every time you saw or pound or plane. 

Wood-working For Beginners, C.G. Wheeler

I bought this workbench as part of my first tool purchase a few years
ago. It's $140 at Harbor Freight and it is really handy. The drawers
are nice to have. It's a good size for my shop. I have found ways to
use the vise and bench dogs. Unfortunately it's really light and has
no rigidity to speak of, on account of it being bolted together with
wimpy hardware. It shakes and bakes under any pressure at all, be it
from sanding, planing, poking with a fingertip.

After having so much fun with threaded rods on the past couple
projects, I finally decided to do something about the bench. I added
2" feet to raise the work height to 36".

I routed channels through scrap 2x2 and glued one to each leg with
5/16" rods tying together the trestles and feet--cross dowels at the
top, nuts and washers counterbored in the feet. That size rod was
chosen simply because that's the largest size cross dowels I could get
at Lowes. I thought I could pull this off without removing the top (wrong) or I would have went with 3/8" rod and tee nuts on top of the trestles.

Lastly, I ripped, zipped, and gripped new stretchers (resawed, routed,
and reglued), glued those to the underside of the flimsy shelf
stretchers, and ran 3/8" threaded rod. Those have countered nuts and
washers at each end because Lowes was out of the tee nuts.

The result? Success! The bench is now a little heavier and a LOT
stouter. I'm happy. It still fails at a lot of wood holding tasks, but
at least it doesn't rack in a gentle breeze. Between this and my
Parrot vise, I can work better and have bought myself time before
building a real bench.