We live in a world where roadside underground pedestals sit with covers ajar for months because they don't want to spend the money to fix them. I wouldn't lose too much sleep over it.
I've seen them go years with no screw, just a zip tie. I dropped my own in the snow years ago and just put in a stainless or brass phillips head until the spring thaw. Had at least two techs out in the interim and none of them said anything about it or assessed any fees.
But you may be able to buy one. Security Hex, Pin Hex, I think ... Stainless?--
Find your USNG coordinates: