I made my own brackets. One under the dash attached with bonded T-nuts that held two speakers (I think they were 3.5" dia speakers). Then I also made two additional speaker brackets, one for the drivers side and the other on the passenger side, which held one of the same speaker on each side.
I posted pictures of my install on this site several years ago. Let me know if you're interested or have trouble finding them. Stereo - Secret hidden installation
http://6066gmcclub.com/forum/album.php?albumid=30