The good news:
I found the presumed-missing part of the Gorilla workbench drawer kit. (You'll laugh: I'd cunningly hidden it in its correct position under the workbench top.) So the bench is now complete, except for a bottom to the drawer ... which is going to be a little bit of a problem, because the drawer bottom needs to be 17.25" x 56.25", and I can't get anything longer than about 48" into the trunk of the car unless I can fit it in diagonally. (Even if a 17" wide by 56" long board would fit diagonally, I'd never get it in through the available opening.) Also, since the longest project boards the hardware store has are 48", I'm going to be forced to either buy an entire 4'x8' sheet and have them cut it for me into pieces that'll fit in the car, wasting most of the sheet, or make the drawer bottom from at least two smaller pieces. (I'm not sure whether the best approach is three 17.25" x 18.75" panels siude by side, or three tongue-and-groove boards¹ laid lengthwise.)
I did, however, manage to pick up a good 56" workbench power strip and a shoplight pretty inexpensively in today's Lowes run. The power strip was the most expensive single item, which should give an idea how inexpensively I got away with everything else.
(Update: The shoplight turns out to be surprisingly nice. Not only does it have a reflector design that's roughly 90% efficient at putting emitted light on the work surface, as compared to the 54% of most traditional fluorescent shop lights, but I found "sunlight" — 5000K color temperature — tubes for it at no extra cost. And as if that wasn't enough, it also turns out the 32W T8 tubes it takes are instant-start. Click, and there's 5600 lumens of 5000K light on my workbench.)
The [minor] bad news:
I looked at the workbench frame again and there's no way to add a second shelf using Gorilla Rack parts, because there's no mid-leg set of cam slots for the beams. So if I want to add a middle shelf, I'm going to have to fab it myself from scratch. (Not that this is a big problem at all. I already know where I can get 1" aluminum-alloy angle rail that'll make perfect support beams.)
Now, off to go wire up the shop light....
[1] Two 6" and one 8" board should work out just about right. "But wait," you say, "six plus six plus eight is twenty, not seventeen!" And you're absolutely correct. But, advertisers and the lumber industry being what they are, nominal 6" tongue-and-groove boards are actually 5" wide, and 8" boards are 7" wide. And we have always been at war with Oceania.