Adding a tarp to a complex shape like that won't be very successful. when you are really going to want a fly...wind and rain and cold...that flat tarp will pop and jump. Leads to wet inside the tent.
Without a fitted fly all you have is insect protection. It sure doesn't take much to Google the manufacturer and find out if there's a fly available. If you don't have a smart phone five people within arms reach will...enroll them in the treasure hunt! Unless they're bidding on the tent!
Heck...it could be a Mountain Hardware, Marmot or some other high end tent. Buying it at the right price then spending time searching Craigslist via:
http://www.searchtempest.com/
...might get you a deal.
Tom
