I have made many, many of these.

First the vaults do commonly use a five sided bolt to secure the
hatches, just like fire hydrants use on their caps and on the
operating nut (the one on top).

The Key being discussed is most likely a mainline valve key which will
have a square receiver that will slip over a 2" square nut. The 2"
square nut is the industry standard for operating nuts on buried water
main valves. This is true for butterfly, gate and knife valves. The
only deviation is inside of a vault or pump house when a hand wheel is
used.

The pick end on the handle is for prying the lid or cover off of the
valve box in the street. DO NOT DO THIS!! BAD! BAD! BAD! Authorized
personnel and all that rot. That said I have made them (mainline
shutoffs) mostly of 5/8" cold rolled steel. You can use 1/4" flat to
make a box end that will fit over the 2" square nut or go to your
local supplier and buy 3" of 2"+ square tubing w/a wall thickness of
3/16" or 1/4".

The shut off by your water meter is normally a slotted affair and only
about 18"-24" long. They are normally in two sizes. One is 5/16"
thick and the other is 3/8" thick. Unless your utility has given you
permission or installed a shutoff on your side of the water meter you
could get in trouble for even touching "THEIR" valve. Again having
mentioned that I make them out of 3/8" cold rolled steel on a wooden
jig (for the tee) the slotted end I always make in the thicker size
and 2" long. I have heated and bent flat bar for this but it is a lot
quicker to set up a wooden jig and cut all your pieces then just do a
mini assembly line. They do not have to be thimble tight on the
slotted operating nut. Just remimber "Righty Tighty, Lefty Loosy".
Todd