Gasket wouldn't hurt is pretty easy to pull the valve cover on a Miata. ;)
Here is a how-to (With Pictures!) - http://www.miata.net/garage/valvecover/index.html
From the Archives here is a shot of my the POSII ('92) sans valve cover!
http://www.harrisonindustrial.com/im...a/IMG_0912.JPG
BTW. Good writeup here on how the HLA's work - http://www.miata.net/garage/hla/index.html