Review on Battery Tender Female Cigarette Adapter Accessory Cable for Battery, Inverter, RV Mobile, Charger - Seamlessly Connects in-between! by Michael Crossman

There are no simpler products and this is reflected in the price. it (most things). I have a 2x USB charging adapter (fused) that I'm using and it works fine. I connected it directly to the battery using the same connector as my charger. The main disadvantage is that whatever you plug in there has a spring to keep it in contact and this will eventually cause the adapters to vibrate. (I had these things insulated in a tank bag and they still fell apart even though I drive a tamper.) Duct tape will help, or maybe you can tinker with thick rubber bands cut from old hoses. or something like that. However, it is worth the money.