Sadly, there is no way of doing exactly what you propose. You cannot have compound documents like this in SharePoint. It would have been logical to just enable attachments to the library, but sadly, trying to do so creates an exception denying attachments for document libraries and surveys.
You may want to solve this by not using a document library at all. Of course, this is not doable if you rely on the library features. If all you want to do is to create a collection of document, look at using content types instead, ie create a 'Proposal' content type to which you can attach files. In that way the proposal itself becomes the item, not the documents.
Another option is to use the document workspace site type, but again, not really a 'list/library' way of doing it.
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