Every year about this time we have our private sewage system (Klargester) serviced and the sludge pumped out. A few years ago their two big trucks managed to break one of my manhole covers, (which they denied having seen) so now I put traffic cones on them so there is no excuse. This year when placing the cones I noticed that two adjacent aluminium 'rodding eye covers' had been cracked at some time - probably not by them so I needed two new covers. I can buy the complete assembly for about £25 but NOT spare covers.
Handy having a CNC Plasma Table - turns out that the covers are true mathematical ellipses so easy to draw in Autocad - would have liked to cut them from 6 or 8 mm aluminium tread plate but none to hand, so the plate of 1/4" mild steel that was on the machine got used. A coat or two of cold galvanising will delay rusting a few weeks (!) but I'm sure they'll be fine for years.