I am trying to make a public share similar to Apple's dropbox where a guest can add or submit files to a folder but cannot open the folder to list its contents.

In samba I've successfully created shares with read only access and full write access but I have not been able to create a shared folder with read only access and a subfolder with write only access. The goal is to have a single shared folder open to public that everyone can freely access with topic headings or folders listed within it. Those sub folders would have write only access and unable to be opened and browse contents. In my samba share I have created the folder structure and ran:
sudo chmod 733 /dir path
for the folder that would be write only. No luck.

Does anyone have any ideas on setting up a share system such as this??

It would be implemented in a classroom where the primary share folder would be a class name and the write only subfolders would be assignments.

samba is running on xubuntu 8 and being accessed from win xp pro boxes