For those who use the foam board, how do you use the sun finder? Find and lock on the sun first and then mount the shield?
As you said, you could first locate the sun and then put the foam board on. This works well, especially if using a tracking mount.
With an alt-az (non-tracking) mount, I've also done it by using a low power "finder" eyepiece and aligning the shadow. By "aligning the shadow", I mean moving the scope until the foam board casts a fullest shadow on the ground. Or, if you look at Paul's first picture, you would move the scope until the shadow cast the end of the scope (in this case by his filter) on the foam board would disappear. With practice you figure out your own technique and it becomes easier.
Another way is to put a small hole in the foam board that lines up with your finder. I've done this before, but now I just align the shadow.
Also, the foam board comes in handy at public events. I labeled an image I took and then attached it to the back of the foam board. This way they can identify any features they are viewing.