Introduction: Insulated Cake Box for Five to Six 15" X 20" Cakes

Wife assigned me to keep five 15" x 20" Costco sheet cakes refrigerated for a nephew's outdoor wedding. Challenge accepted.

Refrigeration Performance

On day of the wedding, we picked up the five cakes from Costco (probably room temperature 70° F since they weren't in the cooler) at 4PM. Put the cakes into this box, used 8 lbs. of dry ice (-109°F) broken up with a hammer into smaller chunks in the middle section, generally not in contact with the plastic cake boxes. 4 hours later at 8PM the inside of the insulated box measured 39°F. I'd guess it would have been cooler if the cakes were chilled to begin with. Probably about 4 lbs. of dry ice remained. Box was stored indoors with 74° F ambient temperature. I'd wager these would have been fine outside in 85° in the shade, but you never know.

Costco Cake Plastic Cases Sizing Errata

The local Costco, in West Michigan in August 2020, claimed their cases were 15" x 20". They are about 5¼" tall according to the wife. I think they are a bit wider than 15" as I had to overlap the "lip" when placing 3 side-by-side.


  • One 0.75-in x 4-ft x 8-ft polystyrene foam board insulation; Very flat is best. My sheet had a slight curl along the edge which made assembly more difficult.
  • Wide tape that sticks to foam board (Packing Tape / Duct Tape)
  • Adhesive - foam board compatible (Loctite PL 3X)
  • Razor/utility knife
  • 4' straight edge (level or drywall square)
  • Possibly Optional: Small scraps of wood for reinforcement of middle section

Step 1: Measure and Cut Foam Board With Utility Knife

Accurately measure and mark the foam board.

  • Use a thin marking device.
  • Be precise.
  • Aligning square, accurate pieces during assembly is easy.
  • Poorly sized, poorly cut pieces are no fun to assemble.
  • I was unsatisfied with some of my edge cuts so I ran them through a table saw to true them up.

Cut the foam board.

  • Don't be a cheapskate. Break out the new sharp blade.
  • It is critical that the cut edges are very clean and square
  • Ensure blade is perpendicular to the foam material so 90° corners are made on all faces.
  • Don't let the knife stray. It will. Focus Luke. Use the force.
  • Careful not to cut whatever is beneath your foam board.

Step 2: Assemble Bottom

The bottom is simply one layer of foam 22" x 48". No assembly required. If this is challenging, stop here, you'll never make it.

Step 3: Assemble Top

  1. Use a generous bead of adhesive caulk down the length of the long edges.
  2. Use the tape to hold it in the place. Don't be stingy.
  3. Do both long edges first.
  4. Depending on multiple factors trim the short ends to fit, caulking any edges where the pieces meet.

Note on Adhesive Caulk

Given that I had less than 24hrs, I chose a caulk with a 24hr cure time. Other caulks had >24hr cure time. Be sure you choose a caulk that is compatible with foam board, as some will chemically melt the polystyrene.

Clean Look

If you are going for a clean look...good luck as the squeeze-out and tape will definitely challenge the aesthetics. Probably can be done, but it didn't interest me.

Step 4: Assemble Middle

The middle is very similar to the top with the addition of the support beams.

Costco cakes are heavy because they are filled with 2.1 megatons of buttery creamy goodness. As I like to over-engineer most of my projects, I added some ~3/8" x 3/4" x 22" long scrap wood the spanned the whole width to support the weight of 2 cakes on the top layer. Notch the long edges to accommodate these if needed. Use a bunch of adhesive on those too for good measure.


Given that the wife has seen cakes stacked 2 high in the Costco coolers (which I didn't find out until after I built this), the necessity is debatable. Really, the whole middle layer is debatable. One could argue that the insulated middle section with gap/vent allows dry ice to be placed in a manner to avoid touching the plastic cake lids. I did have some dry ice sit on one lid. The cake 2 inches below the ice had ice crystals form on the frosting.

Step 5: Seal All the Edges

Consider sealing the abutting edges with tape if you've done a shoddy job like I did with the caulk.

Step 6: Add Cakes and Dry Ice

When purchasing the dry ice, double or triple plastic bag it. That way, you can smash up the dry ice into a few smaller baggies and position the chunks around the cakes pans. It's good for the ocean too.

If you like your cake wet and soggy or don't like the bride, use lots of normal ice.

If you want some nice frost burns on your fingers, touch the dry ice. Otherwise, do as sensible people do, use gloves or have someone else do it.

  1. Add the cakes.
  2. Break up the dry ice, stack/assemble.
  3. I actually added some slab ice packs (like you get in those mail-order
    meal kits), but I don't think that mattered much as the dry ice is 100° cooler than my deep freezer.

Step 7: Seal the Section Edges

Tape/seal all the edges to keep the cold in. This is extremely essential to keep the cold in.

Step 8: Transport Tips

Don't ruin the day. Take care during transport. We lifted the box containing 5 cakes using three people. Take extreme caution to support the full underside/bottom during movement as the foam board isn't structural and may break.