That's not even the right hubcap for a '65... What you have on your car there are '67-'70 Wire Wheel Hubcaps. The hubcap itself is the same as the '65-'66 Wire Wheel hubcap (Sprint 200 Version), but they have a Blue Ford Crest center with a Flat Chrome Spinner. The '67-'69 Wire Wheel hubcap (Which is what you lost) is the same cap, but it has a Red Non-Spinner Center piece.
You can get the EXACT REPLACEMENT cap you need at NPD, or HUBCAPS.com Just ask for a '67-'70 Mustang Wire Wheel Hubcap with Red Center. Price is anywhere from $75 for a used one to $130 for a New one. If you ever want to remove the '67-'69 Round Chrome Pieces with Red Centers from your current hubcaps and add the correct Spinners... You can buy a Set of Four Correct '65-'66 Spinners with Blue Ford Crest and swap them out for your Red Chromed centers and you'll have the "correct" caps for your car. The set of 4-spinners for '65-'66 goes for about $200.00 from NPD. The cap itself was the same from 1965 to 1970, Just the center and spinner changed..
Picture below is the same exact cap that you have, but with the correct '65-'66 spinners.
Also, Noone would rarely steal just one hubcap... Chances are that the cap was not secured correctly and you lost it turning a corner. If the anchoring tabs are not correctly bent to the correct angle, or the cap has been removed 1000 times, It will come off if the metal tabs aren't in the right position. With that being said, In 30+ years of drive, I've only had one cap come of once....just once. and I luckily saw it come off in my rearview mirror...and I was also driving like Steve McQueen at the time when it happened (Don't we all??)... lol...
Years ago, They used to sell Cable kits for the hubcaps that locked onto one of the lugs.. I haven't seen them sold in at least 20 years..Probably a good business opportunity....and they are a joke to make on your own..