Can a through bolt axle of the same width be used?
My searching has at most given me at most Allen Key Skewers, which are still easier to open than with a wrench
Relying on the hypothesis that a thief can have allen keys but not a 15 mm wrench to unscrew the bolt-on axle is rather weak. You should certainly expect a thief to have common tools with them, and to be very fast with using these tools.
I am sure that in your incident, if the thief had less exposure, all the bicycles on the same rack would have lost their unsecured wheels.
There is no easy way to replace a QR-axle with a bolt-on axle (nor with through axle). The latter one (ø9–10 mm) cannot be squeezed into the hollow QR-hub hole (~ ø5 mm); you'll need a different wheel hub that has a solid axle. Then, the dropouts of the fork and frame, if they are designed for QRs, may be a bit too narrow to fit bolt-on axles. They can be filed to be wider of course, but that is irreversible.
That said, I do happen to have a "bolt-on" rear axle replacing the QR. It is threaded all along its length, and uses two large nuts with matching threaded holes to secure it. I use it not for security but because I need a wider rear axle. It is fully custom however and very hard to obtain for two reasons:
- The rod itself must be of a hardened steel, which is typically not available in hardware stores in form or threaded rods with required diameter.
- The nuts must have wrench flats for larger wrench (15 or 17 mm) but have a tiny threaded hole (5 mm), which is also not something usual.
I don't want something that is harder to open (i.e., a pitlock)
Sorry, but you are contradicting yourself. You do want something that is harder for the thief to open; otherwise why bother? There are many "secure QR"-designs on the market, from those with custom heads, to gravity-based ones (can only be opened when a bicycle is upside down). Another alternative is to actually lock the second wheel with a secondary lock to the frame in addition to the primary lock that secures the frame and the remaining wheel to a stationary object.