This is a new battery enclosure for the Homelite lawn mower. The original housed two 12V SLA batteries, the new enclosure is sized to accommodate multiple Lipos. For more details on the upgrade, you might check out this write-up:
https://mbhuff.wordpress.com/2014/11/03/converting-my-homelite-cordless-mower-to-lithium-power/
I've posted the Solidworks part file to Thinkgiverse if you'd like to make changes of your own.
https://www.thingiverse.com/thing:1844850