You should use the proper one. You should be able to find one easily and cheaply from a junkyard.
http://www.nix99.com/ Check these guys out there a Toyota only junkyard.
I tried to search their inventory, but I couldn't come up with anything. If you email them or give them a call I'll bet they have one for less than $30.
You could use something close, but then you have to worry about it not fitting right and how to secure it. Plus if the plastic isn't the right kind it can corrode or melt from the heat.
Good luck in your Search!