Why'd you do that?
By the way, I noticed for those two vertical plank braces on the back wall (visible from the back of the van), you had them oriented "flat". They will provide much more structural strength from the same piece of wood by orienting them "tall." For example, if using 2x4s, attach the 1.5" sides to the panel you are bracing. Imagine laying a plank of wood across two sawhorses and standing on it. If you lay it flat, you are going to get a lot of flex. But if you orient it vertically you will get much less. Another example would be trying to bend a butterknife holding it flat and holding it along the axis 90 degrees from flat (along the "thick" plane of the knife). This is why rafters in houses, I beams, buttresses, and such are oriented in this manner; it is effectively getting the bracing value of the thicker side of the material for the weight of half (or much less) what a square shaped cross sectional piece would get.
If you're going to be adding many sheets of MDF to the back wall anyway, though, I wouldn't worry too much about it. You could always add an "X" to each internal wall, especially the sides which seem thin in comparison to the front and back though. Don't know how that would affect the box performance besides lowering volume a bit, but structurally it would add a lot of strength.