Ron, I see I'm going to have fun with the top.
Don,
I would suggest building the wishing well in a barn or garage where there is power if you have either available on your property.
Since your culvert is 32" you might use a piece of 48" as the main frame. You could lag or bolt 6 x 6 vertical posts for your roof to the inside of the 48" culvert and
still have plenty of room to slip it down over the 32". You could weld pins to the outside of the culvert for hanging cover material like you did on the bridge. Might need to use artificial stone for weight control. Be careful if you weld on galvanized to have plenty of air as the fumes could make you sick. You could probably just use bolts stuck out with the heads on the inside and have the nuts down in the low ribs of the culvert rather than weld, if you enjoy drilling holes in culvert steel. Having the outside diameter of the well being 4' plus whatever facing material you use, stone or wood, gives the opportunity for a higher roof to be in correct proportion. You could also hinge one side of the roof or both at the peak and use a couple of the little captive gas cylinders like they use on car hoods and lift gates, ( even on CNH tractor hoods ) so the roof would be out of the way if you wanted to get a bucket of water.
Sound like a winter project.
Ron