The Low Cost Cruise Missile (LCCM)

How to do-it-yourself, potentially for $5,000 a pop.

...It would not be difficult for terrorists to build their own relatively sophisticated cruise missiles using off-the-shelf components and materials.

What does he suggest can be done about this threat?

Unfortunately there's probably very little that can be done to reduce the magnitude of this potential threat.

Doubtless a country such as the USA has the technology to shoot down an LCCM -- but there's a very good chance that multiple launches from dispersed points relatively close (say 100 miles -- representing a flight time of just 12 minutes) to the intended target would likely make it impossible for a defending force to deal with all the LCCMs launched.

A small (say 5-foot long) target travelling at 500 mph, less than 200 feet above the ground while following an unknown and deliberately erratic course becomes an extremely difficult target to hit -- even if you're ready and waiting for it.

Given that, with a radioactive or biological payload, it would only require a single LCCM to reach its intended target in order to inflict a damaging blow, the capability of defending against such a strike must be considered just modest at best.