In a nutshell, Lowes and HD did sell those blocks. Had I known, all I
had to do was mount the block first, then drill my 4" hole in the
block, then mount the vent on the block.
I did improvise. I put foam blocking to bring out the vent, then I
used the blocking over the opening , then I installed the vent on the
block. I'll try to take a pic tomorrow and post a new thread.
Thanks for everyones help.