Kubota L3830GST, B7500HST, BX2660. Formerly: Case 480F LL, David Brown 880UE
I would mount a 2-3' wide piece of 3/4" plywood to the purlins and make it tall enough to get up to the top of the wall so that you can run wires and/or conduit down it.
Not sure what kind of building this is. But i have done similar by installing a 24" x24" piece of 3/4" plywood on the horixontal 2x6 and mounting panel. Then i run all wires up to ceiling height inside 3/4" schedule 80 PVC pipe.
2009 Kubota RTV 900, 2009 Kubota B26 TLB & 2010 model LS P7010
Since you have the wood purlins, I would get a piece of plywood about 3 feet wide that will span the distance between the two purlins so that when installed the breaker box top will be about 6-7 feet from the floor level.