Locking gears to remove starter clutch bolt

What is the best way to lock the starter clutch so that I can undo the bolt and check the innards?
Honda had a special tool but I don’t have one of those.

I drop a piece of soft plastic/nylon in between the gears to lock them and prevent rotation. a big soft washer is ideal, as it deforms and locks the gear teeth without damaging them.

I used to use copper coins but in Australia they are getting hard to find.

Same here, I use a 2p coin. Soft enough so it doesn’t damage cogs but large enough to lock the starter clutch up.