Hi CJ,
I did this recently. What your picture shows is correct regarding the space available, it was a pain to get all of the components in.
I think the technique I used was to install it to the point you have done and then pull the head of the bolt towards yourself a bit so it's at an angle. Try and fit the washer in from the far side as far as you can into the "wedge" shaped opening. Then try and move the bolt back into alignment dragging the washer with it. I had to tap it into place and also use some small pick tools to get the rubber seals back into shape.
To be honest it was an ugly job I made of it and I am sure there are better techniques but it worked.
Don't give up. That washer has to go in there.
Regards
Peter