The only way i can see to do this is to screw down though the unit
from positon D?? (which i dont want to do)

You could pack out 'X' with a bit of scrap wood and use long screws.

UUUUUUUUUUUUUUUUUUUUUU
------+--++--+
l l|SB||FI| -------| long screw
l l+--++--+
l ----
l l
l l
----------

SB = spacer block cut from scrap wood
FI = fixit block

Christian.