Regarding Lynn in Washington’s pointer to the YouTube video (as representative of a “secure” lock?), I know of, and have known of, for quite some time, two ‘other’ locks which fit, much better, the criteria of “round” and “disk”.
The first “round” lock is manufactured by the American Lock Company. (Yes, that is its real name).
Many truck-delivery companies (in New York City, and surrounding vicinity) use this lock on the back doors of their vehicles, and anyone who, honestly, believes they can open this lock with a bolt-cutter is being dishonest with themselves. Picking is also rather difficult.
The second lock (“disk”) is manufactured by Abus, and is known as [the]”Discus”. I owned my first one while in High School during the late 1970s, and it never failed me (amongst a dorm full of amateur-lock pickers, this was quite an achievement). As seen in the illustrations, it would be extremely-difficult, if not impossible, under ordinary conditions, to push a bolt-cutter in to cut the shackle of a Discus lock..
That’s all here. You have a very nice blog. – Manatee