Makita 18V LXT battery to 12V 15A cigarette lighter type socket to use with accessories like mini portable water pumps and kayak inflation blowers etc. There are better sockets I know but this is what's on a lot of common portable items so I've gone with it for this.
It uses a buck converter board from ebay with adjustable output voltage which I set to 12.8V and gives up to 15A continuous output. I removed the original finned heat sinks from the FETs and connected them to the aluminium infill panels with copper heat paths. I've yet to have a problem with this arrangement and nothing has gone pop. Make sure any FET insulation boots are intact and not conducting to the aluminium. You might need to move the internal stand-offs to suit your board.
The 12V socket is from ebay and is glass filled nylon and about as high quality as you get for a socket like this. I cut it's spade terminal connections off and soldered directly to the socket to reduce the height of the completed package.
I used a Makita OEM connector insert (the yellow piece), part number 643852-2 (approx cost £4.50 for this part on eBay).
The rocker switch works on the main 18V supply from the battery and turns off the complete circuit board.
Stainless steel self tappers hold the lid on. Be care tightening them up to not break off the uprights of the main body. The aluminium infill plates help to resist the torque here.