Been thinking about how i am going to square the gantry up and thought of an idea using the method used below (watch from 2:20)

i know that method won't work on mild steel box section but what if i used epoxy resin on the side and wait for that to fully cure which will then give the side of the steel box section a really flat surface and then do the method in the video? i would need to do this before welding the top rail beam to the frame off course.

what do you think is it over the top?

https://www.youtube.com/watch?v=YOVHtjXi07Q