Just trickle charge using the stock EVSE and the 120V or 240V receptacle of the portable power bank you've just bought. The extender pack that Muxsan sells looks like a well designed product, but there's no way you can DIY anything close to that good of a range extender.
Save your money and be content with the solution you've already purchased. Honestly, it's really unlikely you'll ever use it for charging your LEAF as the car gives plenty of warning before you reach turtle mode.