Detroit 453 oil seal replacement

[Spartan]

Anyone got a trick for holding the cam in place to get the balance weight nut on and off.
I need to replace my cam oil seal.
I tried holding everything still with the crank bolt, but saw the cam starting to flex while everything else was holding still.
Don't want to break a timing gear in the back of the engine.
Manual says to jam a rag in between the timing gears in the back.  That works great if you got it disassembled on a stand, not in the chassis.

Can I remove the rear round inspection cover and hold it there as a last resort?
Thanks

[central pa]

I wedged a pry bar in the pulley rag in the gears works but can be a pain to get to

[Spartan]

The back side of the pulley?

[central pa]

Yes ours had lip on the back you could wedge the pry bar in
First time I did I used the rag in the gear method it ate it . I bunched it up a little it held I was afraid the shafts the gears were on

[Spartan]

thanks