An asst surgeon on the field would be limited to what he could carry in a haversack (pocket kit, tourniquets, bandages, an opium plug or pills etc) and what his "knapsack carrier" would have in the hospital knapsack or field medical case. The panniers would have been no closer than the field hospital. The field medical chest you link to would probably not have made it to the field, as a hospital knapsack ( like the McEvoy Knapsack) or field medical case, would have been more convenient to carry and hold as much or more. Dixie carries both of those items or check out the cases that Trevor was talking about.
your obedient servant,
Surgeon, 2nd Brigade
"not really a surgeon, but I did stay at a Holiday Inn Express"