I'm considering a strap from http://rubberb.com/ specifically one from the Structure Series. Another candidate is one to fit the Explorer I. These are not cheap but they appear to be of good quality/craftsmanship - anyone around here have seen these in the flesh?

 

No experience with Rubber B but have you considered Everest? The rubber strap for my BLNR is of very high quality. Looks like they have a few good options for the Explorer 1 as well.

https://www.everestbands.com/collections/rolex-explorer-i-214270

 

@alam, I purchased a Rubber B for my Explorer II. Very well made, excellent fit and I am very happy with the purchase.

edit: I bought the style that uses the clasp from the original bracelet.

 

I bought a Rubber B for my Explorer II polar, the black rubber with red insert. With tang buckle. Very comfortable and soft rubber, the red insert matches the red GMT hand nicely. Have it fitted on my SMP 300 at the moment with some skinny springbars, also a perfect fit.
Also have one on my Submariner, it uses a Rolex buckle from the original bracelet and works fine.

 

Not from the Structure series, but I have a black rubber for my 216570 Explorer 2. The strap works with the Oyster clasp.
Expensive for a piece of rubber but very good quality.

 

I bought one from a well known dealer in London last month and after about half an hour they gave up trying to fit it and admitted defeat. It was the M103 model, which is supposedly a fit for several rolex models including Submariners, Explorer I / II, Milgauss, GMT I / II. A quick google since shows a lot of people have struggled to get them to fit. The problem seems to be that the tube for the springbar doesn't line up exactly with the holes on the lugs, and being a solid end piece there's no way to force it. They actually tried a second strap in case the first was faulty, but no joy with that either.