Yo Dave. Neat project box beetle. My first one was similar to that, except it used a plastic one.

You're wise to test it with spinners but there will be a difference between rubbishy cast aluminum that will almost certainly crack and legit flat grade box or plate alu.

Here is an idea you may like... Grab some large thick walled alu rectangular box section (you can get it in near perfect short lengths for very little money) and then use some ranglebox to attach a top and lid to it? could be right up your ally (alu?)

I got a 50mm length of 6" by 2" 3mm wall 6082 extrusion for about £3. Maybes some food for thought on boxbot.