I'm a pretty new boat owner, been rafting on others boats for a couple years, and just bought/built out mine at the end of last season, so feel free to take my advice with a grain of salt.
I rock a couple 50 cal ammo cans strapped to the decking on my cat. I just run a 2" strap through the handles and then down through the frame. It works great they are very stable and reasonably easy to secure/remove. Obviously rocket boxes are much bigger, and honestly I'm not sure how stable you could get them up on the decking with just a strap. You might have to build in some sort of short wall coming up off your decking. You could get it stable if it's lying on its side, but if this rocket box is holding a groover, I wouldn't recommend that.
In short, I'm not sure how to make that work well. I carry a rocket box in a DRE sling that fits into the front bay of my frame where my crew sit. Just having one there doesn't interfere with their leg room at all, and it even gives the passengers a little table in between them. I haven't tried it yet, but I think you could even put two there and still have adequate room for feet.
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