also not gonna lie, I found this by not knowing there was a race to do and thinking I had to enter it from the bottom.

I found a clip in the industrial district that could be useful in 100%. You need to go to the floating crate in the ocean and switch to elastigirl. Then swim under the container to around the middle part where the gold brick would be and switch to boat form. Get yourself lined up where the boat is going across the short side of the crate, and switch out of boat form and you should be inside the crate. To get out of the crate simply switch to the other character.