Welding cast iron manhole covers

Spec
I know you already made up your mind based on the suggestions. But worth mentioning another solution...
What if you build a metal door (or wire steel wire frame) like on the hinge either swinging up or down, have the hinge bolted under the outer ring of the manhole with a latch to the opposite side. This way the manhole cover will only be for cosmetic reason with support beneath it, (epoxy or weld the cover the way you want), and once the cover is lifted the hinged door can be lifted and manhole serviced!

Sorry I am late. I thought maybe you could drill part way through underneath, then thread and use a stud. Use that to bolt a plate to the under side? I stick welded the arms on a cast iron bench, really didnt go well......... I just thought hidden studs might be useful, but not sure how easy cutting a thread into cast would be.