It there is room at the bottom of the box, and the leads reach, why not? I once installed a 3 1/2" drive in a 5 1/4" bay because when I bought a convertor it was the wrong type. I screwed in one side and put a great wadge of Blu-Tak the other.
Which brings me to my other point. If you can use it without it touching any vulnerable parts of the HD, use Blu-Tak to act as a physical buffer so that any knocks aren't transmitted to the HD from the case.
Another thing is that some HDDs have screwholes on the bottom so that you can mount them on the bottom of the case, it's a question of whether there are any holes on the case, and if you have to drill some, you'll probably have to remove everything and put it back later.
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