When our church needed a way to store and set out our metal flatware (we did not want to keep buying plastic ones), I set out to fill in the gap with cardboard. I began by forming the cardboard into the shape that I wanted it to have. I cut out a long rectangle and then drew lines dividing the space into sections for forks, knives, spoons and two places for napkins. I cut the dividers from more cardboard and used glue and duct tape to hold it all together.
Now that I had my base form, I set about to disguise it. I decided to use the same approach that I had used on my cardboard tray. I ripped one to one-and-a-half inch pieces out of scrap cardboard and soaked them in water for three minutes so that the glue holding the corrugated section in place softened and allowed the cardboard to separate. I drained off the water and removed all the darker corrugated papers I could find. I blotted the excess water from the lighter side pieces, leaving them damp, but not dripping.
With the cardboard scraps in hand, I began to glue the pieces to my form. I allowed each piece to overlap the one before it so that the form could not be seen. I covered my basic form inside and out until all the surfaces (including the bottom) were covered. I used decoupage glue for this to help make the storage unit water-resistant. This did take some time, but it was worth it.
In order to make the unit look a little nicer, I thinned some acrylic paint (burnt umber) and, working in small sections, I brushed it on and then immediately wiped it off with a rag. This caused the cardboard to take on a stained look which was a little better than the plain look of the original cardboard.