Here's a MR that might be of interest to you...  incredibly useful.

http://www.sharecg.com/v/67360/gallery/5/3D-Model/Quicks-Quick-Resource-Meshes

best to go with one-sided geometry with extrusions curving into the areas that will suggest thickness, eye-holes etc. You'll save on polys, have the illusion of thickness, and avoid the problems you are encountering. (easier to UV, too).