You could possibly use a cheap brake pipe flaring tool to do the job.
The flaring tool itself is tapered at approx a suitable angle and then the clamp part could be adjusted to suit.
I don't have mine to hand but having used one many times I can see how this would easily work well if you got the clamp part right and they cost less than £10.
They also have die's which could be used but I can't think of these would be usable or you could make a for if needed.
https://images.app.goo.gl/dKwzxiWtUxEsZjpf8
Johnj
Webley Excel, Webley Tomahawk, Air arms khamsin, Air arms Pro Sport, Bam B26-2, Bam B45-3, BSA Mercury, Sharp Innova (licenced Indonesian version).