You can submit both files (OBJ+MTL in a ZIP file and a SH3F file) if you want!
Submitting the .sh3f version like you did for the window with sill and waterboard model is great because it makes it useful for Sweet Home 3D users until I add the model to the contributions library, and it will reduce the time it requires for me to add it to that library.
The properties you chose are good. No need to specify the value of the properties that are not mandatory or not necessary for the model you submit.
Just adding an ID could be nice to avoid your model to appear twice in the catalog once I'll add it to the contributions library. If you specify an id, please use the same naming convention like I did in contributions PluginFurnitureCatalog.properties file, i.e. yourName#ObjFileName (for example, OKH#window_2x3_frame_sill)
just realising that for instance my categories is unlikely to be to everyone's taste
As the category of a piece is not editable when it comes from a sh3f file, it's not a bad idea to use a name that will help users to find your model. But avoid creating a different category name for each file you submit! 
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Emmanuel Puybaret, Sweet Home 3D creator
