I guess this is the season for rotted basal plates?

This is my largest h.mandonii, which I thought was going dormant after coming inside 6 weeks ago. The leaves started to yellow, so I unpotted and found this:

My question is: once I've cut away the bad area, which is large, what's the best way to maintain the remainder of the basal plate? Obviously, I'd like to keep the offset going until it can establish it's own roots. Do I treat it with an anti-fungal, and do a 'sphag'n'bag'? Will leaving as much of the unaffected bulb as possible help the basal plate/offset? I expect ~1/2 of the bulb gone, with a large exposed surface. This will have to be well protected from fungus gnat larvae...suggestions?