OP and readers; I guess I should make at least a token improvement input that doesn't require a smiley face at the end..

As OP noted, it is a bit tough to cut the cog to allow the washers to fit into it.

Improvement for others might be to cut/fit a bit of scrap 1/16" or 1-2mm sheet steel to fit into the rounded triangle of the cog. After one has one correctly fitted and beveled for welding, then just make more of them and weld them in to the cog followed by drilling holes for the bolts. Rationale is that the total work might be less, doesn't require cutting the original (very hand) cog metal, avoids a possible weakness as off the shelf steel washers aren't very strong, nor are they well supported by the limited weld surface to the cog in the current model --there may be a tendency for them to bend or twist. Hope that helps someone go forward.
/K