"Sold Secure" sleeved chain in long(er) lengths...

[vickster]

https://securityforbikes.com/products.php?cat=Chains+(without+padlocks)

Note that it's not cheap, even if you don't go for the bolt cropper-proof versions

TMN to me

 

[SkipdiverJohn]

Close shackle high security padlocks are the thing to secure objects like gates, garage doors, and chains with as they don't offer the space to use simple brute force tools on them. That means the only way they are coming off is if the thief has serious lockpicking skills or the ability to use industrial cutting equipment undisturbed. Open shackle padlocks can be forced and self-locking ones can be shimmed and should be avoided. I use Squire Valiant and Squire Stronghold, that were bought cheaply as NOS. I don't think the models I have are made any more but are worth picking up NOS.

 

[lane]

If you look on the security for bikes website linked to above they have padlocks which you wouldn't be able to open in the way shown using two spanners because there is not enough room to get spanners in. It's a good website with lots of good info even if you don't buy from them.