Create a table, one column and two rows. Turn off its borders, and set its
row heights to Exact values.

That may be enough protection, if users can be trusted not to change the
setup deliberately. If that's not the case, put a text form field in the
upper cell and protect the document (preferably a template) for forms,
adding a password if desired. To allow inserting a Visio object in the lower
cell, you'll need some VBA. You can download a template from
http://jay-freedman.info/form_­picture.zip that shows how to let users
insert a picture, and it should be not too difficult to adapt it to insert
an object. If you have any questions about it, please post here or email me
at the address in the template.