I also want to say there's a trick here of holding the plate in place, then mounting the leg snugly so it holds the plate "where it needs to be" then installing the screws to hold it in place, then torqueing down the leg bolts. I seem to remember mounting one and having to take all the screws back out for the threaded part to align properly with the bolt holes and bolts.

Tip: Reverse bolts (and screws) until the threads line up with a thump, then commence tightening. Always start in reverse and you'll never cross thread.

I would remove the cone and slide it up the cord, Install the plate, then reinstall the cone over it and attach the cone back as snugly as possible without deforming, cracking or trimming it.