I think your "window" for accessing the chain is too small.

I actually purchased the official MB tool for replacing the timing chains on OM60X engines. It bolts to the head using two of the 10mm bolts from the valve cover. It has a very wide "window" which really only holds the chain to the sprocket at two places about 180 degrees apart. Works just fine.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC