

The paper you linked says “1 microgram is sufficient to trigger one thermonuclear weapon” which corresponds to 6×10^17.
This makes your “few thousand” of by 14 orders of magnitude instead of 15, I bet you feel vindicated now.
For example, a device the size of a hand grenade with tons of TNT equivalent output
A man portable nuke has existed since the 60’s so it wouldn’t be a game changer.



Yes, but if you take the train or a private car you won’t be searched. To get it across international borders try smuggling among other cargo or by submarine, maybe even diplomatic courier.