Step One
Make a series of rectangles as pictured.
I suggest making one, hit CTRL+C then CTRL+F, hold SHIFT, move it down a bit, then selecting that second one, CTRL+C, CTRL+F, then CTRL+D. You can do this ad-nauseum but I suggest between 3–5 for the 70s vibe.
Color them, then select them all, hit CTRL+G to group ’em. Moving forward I will be calling this group “rectangle group.”
Once you’re happy with it, you’re ready to pop it into your Brushes. Be sure to open that by going to Window>Brushes.
Step Two
Select your rectangle group, drag and drop it over to your Brushes Window.
It’s gonna ask what kinda brush this is. Select ART BRUSH.
💀 Failing to select the correct type of brush will result in a brush type you aren’t seeking to create. You’ve been warned. 💀
Step Three
You want to be sure Stretch to Fit Stroke Length is on and the Direction is Right-to-Left or Left-to-Right.
Name it and hit OK.
Step Four
Bask in the beauty of your creation. If you want to adjust something, double click the stroke in the Brush Window, make adjustments there.
Notice it bends beautifully at the corners. 🤤
Step Five
To troubleshoot a flipped orientation start with selecting the stroke affected.
Under the Brushes Window, click the button seen in the image to the left. It’s called “Options for Selected Object” if you hover your mouse over it.
Step Six
Hit Preview then Flip Across. You should see the stroke correct itself.
This works on stroked objects and stroked paths alike.
Last Thoughts (Step Seven?)
You can do things with the stroke including dashed lines and adjusting the width with the Line Width tool, size the stroke (it defaults at the size of your rectangle croup, so either you need to scale the stroke below 1pt or make the initial object smaller).
You will need to manipulate your closed shapes, or potentially mirror and/or strong-arm the paths with duplicates to get the look you want.