I use a 16x16x18 box I buy for about 3 bucks from storage rental facilities. This will perfectly hold a 4 tiered cake and my 4 tiered cakes are at least 24" tall. With some modifications I basically assemble the box (except folding down the top flaps, I leave those sticking up and unbent so they are rigid), I use a cutting of the rubber stuff you put under your rugs on a hardwood floor in the box to keep the cake from sliding around (but only if the cake is on a board 14" to keep it from sliding around in the box when driving. If the cake is on a 16" cake drum I do not put the grip in the box, otherwise it makes it really hard to get said cake out of the box), cut one of the side corners so it swings out like a door, put the cake in it, put a few pieces of tape to keep said door closed, then use plastic to wrap the box, including the top. The box looks jenky but nobody really sees it except venue staff and believe me, they aren't paying attention or don't care because most people show up with a cake in NO box. We can argue hygiene aside, but doing this also helps keep the cake cold for travel, especially if you have the space to put the whole thing in a walk in like I do. I can travel at least an hour to a venue with a SMBC 4 tiered cake boxed this way (because I boxed and refrigerated for at least several hours) and as soon as I open the box at the venue it's so cold in there it's like I just took it out of the walk in. So there is another reason to box your large cakes - I see so many people post about freaking out about traveling with a tiered cake in the middle of summer and asking what to do. In fact, I've even lined the same type of moving box with ice blankets and delivered a buttercream cake 2 hours away to a location I knew was going to be 100 degrees. When I opened that box it was like opening the freezer. Aside from the natural sweating of the ice blanket (which was on the blanket, not my cake!), it was the perfect white trash freezer for under $15, and the entire thing was re-usable.
Then I leave a pretty little 8" BRANDED box at the venue for left overs.
So yeah, when you get into this business you need to be able to bake, decorate, market, do math, know marketing and constantly market yourself, be internet savvy, an engineer and creative when it comes to things like getting your product from point A to B in perfect condition. LOL