Match shooters tend to prefer clear inserts for the front sight, which give a clean crisp "floating" ring unobstructed by metal bars. I've used those for years, but they are only commercially available for the standard sizes of match front sights. I've often wished my HW 55's, for example, had the same thing.
Then a few weeks ago I ran into a gent named Neil Price in the UK, however, who makes clear inserts to ANY SIZE YOU WANT! He regularly sells these on Ebay and will take custom orders. This is truly a godsend for any old or odd-sized insert-type front sight you have lying around.
In this pic you can see three sets that I ordered for myself, for Parker-Hale, Weihrauch, and older Diana front sights. I got sets of 10 different sizes, which Mr Price packages in a standard box, but he will supply any number, any outside diameter, and any aperture diameter you specify.
This close-up shows an insert in place in a Parker-Hale front sight. Note that the edge of each insert is "stepped." This gives a snug fit, results in a crisp dark reflection off the beveled sighting aperture, and ensures that it is facing the correct way. It's not visible in the pics, but each insert is also stamped with the opening size.
Mr Price is delightful to do business with. The service was quick and the price extremely reasonable. I just can't say enough good things about these little gadgets, and I HIGHLY recommend obtaining these for any tunnel-type front sight you use!