Sorry, I meant to mention that. I am going to use two cells to run an old Eureka cordless stick vacuum. It pulls around 5.4A but on startup its a bit more. I only need one tray, but I was going to buy two. These are the cells I plan to use. If this works well, then I might use four cells in an old Roomba (that formerly used 14.4V NiMH batteries). BTW, I do plan to install protection PCBs. I see some options on eBay — but I am very open to recommendations and/or experiences (from items there or elsewhere).
So $9 to build with nylon injection molded and $7 to build with PLA+ 3D printed. Just go for the pretty injection molded one. Either works. If you were needing hundreds of them, the $2 would add up, but not in this case.