Camille Fournet makes the original straps for Jaeger-LeCoultre, Cartier and some other “premium” Swiss brands. They are considered to be one of the best strap manufacturers in the business. Through their website you are able to custom order watch straps and that’s what I did 2 weeks ago.
The original strap for the Jaeger-LeCoultre Deep Sea Chronograph is 21/20mm which I think is a little too obstructing, so I ordered a 21/18mm strap in the same style but in a “sailcloth” material (not available online but through their customer service). The strap was ordered in the same vintage shape as the original strap and looks a lot like the strap that comes with the JLC DSC Cermet.
Since it tapers to 18mm instead of 20mm I wasn’t able to use the original buckle but I was able to buy a buckle from an original tropicstrap from a fellow collector.
A good fit I’d say and I’m guessing the strap is OK to swim with as well.