Copied how The Insulator Store used to make them. 15" by 15" by 6" corrugated cardboard boxes. I think these were $32 from Staples for 25, shipped free to the nearest store. Few bucks for a box cutter. Cardboard for the inserts free, just ask someone working or pull your car around to the back of the store and jump in the dumpster.
Cut 15" by 15" by 6" strips, then mark three lines on each strip at 3.75" apart and cut out a 3" long slice at each marking. Make sure to take out about 2/16" so that the slot can receive the cardboard that will be inserted.
Takes patience which I don't have but worth the time.