Using a 2.5mm electrode on that 3mm box section seams to be working well, did try a 3.2mm electrode and after a couple of short welds it kept blowing the 13amp fuse but I kinda expected that.
Still just using draper electrodes, when I'm near the end of the box I will order some better name ones but so far to my untrained eye they seam to be ok.
I have a couple of different machines, ur next step would be a DC inverter R-Tech are prety good iv used them and the prices and customer service is good give them a phone can usualy offer a machine to suit ur budged new and ex demo machines