sorry i can't add much again, but what i will say is consider emailing the dude your getting the rails and screws off first for a price and pay outside of ebay, this way you will get a better price and while your at it get him to quote you to include and also not include the spindle.. chances are due to combined postage and a big order you may find yourself getting the spindle alot cheaper than anywhere else.. this is certainly my play when the time comes.

also i only just realised your building this machine to work on a boat.. thats just awesome in its own right.

expanding foam is also something i was considering for my build.. it's very cheap and i think it would go some way to take the ring out of the steel as it wouldnt have any air to resonate through the center of the box section