opinions on this please

[suzywong]

"layer after layer of security and fencing that shrink slowly "

should it be shrinkS slowly as the subject of the verb shrink is a singer layer?

singLe layer, not singer

easily done but then I don't get to hand this post to a sub team on the Independent on Sunday as the author of the sentence in question presumably did

Don't know really but my instinct would be shrink as there are apparently two things shrinking (security and fencing).

IMO 'layer after layer' suggests the plural layers and therefore shrink is correct. However, I do think the sentence could perhaps be rewritten as it doesn't read well.