Here is a cheap focuser:
http://www.surplusshed.com/pages/item/l3630.html
Making a finder is more or less as you describe. You can improve it with matte black paint inside, perhaps a baffle, and a dewshield. Surplus Shed also has cheap diagonals. You can also use a postal mailing tube as a lightweight alternative. It just needs to be painted for waterproofing.
Phil