The connector is already there, as it's a fixed recharging point I'd screw it to the wall and hardwire it with a bit of the left over cable. Don't need a diode I checked polarity. Have I spent anything yet? Ah, 4 no4 screws and a bit of chocolate block (or whatever floats your 12v). No. Not £9.
what I get for the extra £5 is an ugly box I can't mount, 2m of cable I don't want a cheap switch to go wrong lots of useless tips to clutter up the place. And, the need for another 12v cigarette lighter socket and fitting.
I know which solution (does) suit me. I have a few similar running odd voltage stuff.
Being me, I'd also try whether it runs straight from the 12v
David Mack's advice is spot on.