Edit 4/5/19: In addition to the larger RC battery type, I have now uploaded a smaller case for the Mini Brick that is more common to airsoft batteries. If the dimensions do not suit your needs, all you need to do is change the parameters of Length, Width, and Height that are now included in the fusion file for your convenience.
A simple box I designed for a Duratrax 7.2V 3000MaH "brick" battery in order to mount it to my AK Masada. The box will fit any battery of a similar shape with maximum dimensions of 135mm long, 45.3mm wide, and 23.2mm tall. For mini bricks, the max dimensions are 121.3mm long, 35.8mm wide, and 19mm tall.
The case was a tight fit on my ACR rail, if it has a looser fit there is an optional screw hole to clamp the box down using one M3x35 screw and a nylock.
The box has no extra space to hide the wires, though I have added a simple cable clip to keep things tidy. They are sized for two conductors of 14AWG or smaller. The rectangular cutout on the lid will fit a standard Tamiya connector.
You will need 2 M3x8mm screws to attach the box cover. This cover has a rectangular hole sized for standard Tamiyah connectors.
There are currently two versions of this box posted. The standard version is just the box, the "Omega" version is a custom I did for my gun, featuring the greek symbol Omega and two tridents. Both versions are otherwise the same.
Fusion Archive and .STEP files included for your convenience ;)