Choose a dump truck with a really big trailer and wash thoroughly. Check thrift stores and garage sales if you don't have suitable selection at home. My dump truck trailer measures about 9" x 6", roughly half the size of 9" x 13" sheet pan.
Bake your cake and cut to fit. If you want a layered cake, depending on the depth of your truck, you might have to bake two thinner layers or one bigger cake that you cut two pieces out of. Use as big of a spatula as possible to gently lift the layers and place them in the truck bed. An extra set of hands won't hurt either.
Even if it breaks a part a little, just put the broken pieces in to fit.
Add filling if desired. I layered some cool whip in the middle and placed the other half on top. You can see in the photo how that top layer crack in half, but I just squished it back together. No one will ever notice!
Frost with chocolate frosting. Since I was using what I had at the time, I only had about 1/3 can of chocolate frosting from a previous baking project, so I just melted that down and poured it over the top.
If you really want to make it look cool, crush some chocolate wafer cookies and sprinkle them over the top. A few gummy worms poking out here and there would be fun as well. I was not able to do this because of the extra ingredients it requires, but just love the idea.
Prepare to amaze your little trucker!