The "setscrew" will have to be threaded M14x1.0. Not real common.

To the OP: Shimano's "mushroom" tool (TL-FC15) came with every aftermarket 105 Octaling crank set so any LBS should have a surplus of them. BTW, the rim OD of this tool is 0.650" so if you can find a washer that size it should work too. Actually a dime (OD=0.700") is a bit too big.