Hey, good car
My sugestions.. go to local junk yard and get a new dash cover thangiemabobber and cut it the correct size, be alot easier and cleaner looking then some how making it look good by rying to fill in gaps.
As far as securing it, well, i havent figured out a way since there is about a 1 inch gap between the sides of the stereo, what i did with mine was secured it with the tangs on the radio sleeve thangie to the cover, and then screwed it back on, best i could figure out. been like that for about 4 years. and then for my eq i just cut a hole bigger in the cover, and electrical taped it to the stereo by gouing around and around and around and around
I mean if someone really wants the stereo, they are going to get it no matter how good its screwed it, just a matter of an extra minute of them ripping it out.