It will be difficult finding an intact lower compartment that isn't just as rusted as the ones you have. Within the last few months someone was looking for a replacement compartment and managed to fine one from another member. I have not heard of any others being available.
A seller on ebay that goes by the name runamuck12
dismantled a damaged 310 several years ago and was listing those parts on ebay.
A seller with the email address of Lidgej AT gmail.com
was selling 310 windows on ebay at one time so they know of some compartments.
My guess is it would cheaper to just rebuild the existing compartments. That's what most of us have done.