How to create flat tar archive

for GNU tar you can use the --xform argument. It takes a sed expression and applies it on each file name. So for removing all characters up to the last /, you may use:

tar -c --xform s:^.*/:: your-files

outfile=$(pwd)/ps.tar;find . -type f | while read file; do 
    tar rf $outfile -C $(dirname $file) $(basename $file)
done
gzip $outfile