This is an end cap sized to fit the 4" PVC fence posts you find sold in most US big box hardware stores. It has integrated 8-32 (M5 will probably fit too) nutraps used to mount the cap to the tube.
I designed this for use as a LiPo battery tube for my electric bikes and added mounts for Anderson PowerPoles and a DB25 (db25 is for cell level monitoring/balancing). I've included both the customized mount version as well as a "blank" cap that can be modified using the supplied sketchup file. There's also a "drill template" file that makes drilling holes in the side of the tube a lot easier.
The plastic bits holding the electronics (these are cell-log 8s, used to monitor battery voltages at the cell level) is another project entirely, I'll upload and link that one separately.