Go to your local Big Box hardware store (Home Depot, Lowes, Menards, Ace, …).  Find a nice employee in the lumber section and ask for "Four cheap two-by-fours, and someone to cut them for you".  Getting the boards cut in store saves you time, plus the cuts are straight and accurate, and it's easier to fit in your car.

Have them cut the boards for you to these specs:

• Board #1 and 2: 20.75", then 20.75", then the remainder should be cut in half (will be 27.25" each, minus whatever is converted to sawdust)
• Board #3 and 4: Just saw these in half: 48" long.

This will give you four 20.75" boards, four 27.25" boards, and four 48" boards.

Go back to the store and pick up the screws you forgot, dummy!

Screw them together.  See the Sketchup plan (attached).  The 48" boards are obviously the "rails" and support posts.  20.75" boards should link the rails together to form front and back panels.  Squaring things up may be a challenge, so try to match them up against something you know is square (a table, a tiled floor, etc) before putting two screws into each.

Now link the two panels together with the 27.25" boards, which completes the rack by giving some depth.

Bling the heck out of it!  You can stick with the bare wood "industrial" look, or give it a coat of paint, some wheels, a shelf or two.  Make it yours.

Add your gear.  Use more wood screws to secure faceplates to the wooden posts.  For small items like cable modems and wireless thingamajigs, you can put wood screws into the side and hang them (most of these have special slots, as if they were... somehow… made to be attached vertically…)

Should the rack posts wear out from excessive screwing, turn it around.  The design is fully reversible.  You can only do that once though, obviously.