Some of the pictures of "stuff" on the hoods of WW II Jeeps are pretty scary. Like a casualty (or 2) in a stretcher laid crosswise on the hood. I don't suppose the jeep was driven very fast that way, but still it looks pretty precarious.
Another one shows a long range desert group (LRDG) jeep with several jerry cans lying on their sides on the hood. You'd have to have a lot of faith in the gaskets to put something like that over a hot engine, but priorities change in wartime and safety moves way down the list.