Where to find a custom size battery box

I've fitted in a second house battery (group 27 bat) for my pup, and instead of trying to two box covers, I'm thinking about trying to find one that fits over both batteries and then using a pair of wrap around straps to anchor the box to the pup frame.

The dimensions are 27.5" l x 7" w x 9.5" d.

I was thinking about looking around for toolboxes that were close to this size and then taking off the top of the toolbox.

Does anyone jumped this hurdle before? I don't want to re-invent the wheel if there is something out there that is ready made, can take the outdoors (sunlight/temp/precip).