A mute like the metal ones that follow the flare of the bell could be made, but the math behind it would be complex. If you make a mute with only straight lines in the profile, then we are down to pretty simple geometry, which you should base on your own measurements.
The simplest pattern is that of Humes & Berg Stolelined mutes, which is a truncated cone. The Ion Balu mute is a combination of two truncated cones. As my math English is very limited I will leave the math to you.
My experience with straight mutes says: the lighter the better. That goes for ergonomics and for fast response. But of course the structure shall be strong enough to survive normal usage.
Experience says that mutes raise the pitch. If they protrude sufficiently from the end of the bell then that tendency diminishes.
Tom Crown, the mute maker, once told about making a bass trombone straight mute, I think for Mr. Kleinhammer. TC wanted the mute structure to be strong so he folded back the metal at the opening, so that there would be a double layer at the end of the mute. However the low C right above the pedal would not speak at all. TC tried then to avoid the folding. The sharp edge was supposed to cut the airstream in a cleaner way. The Low C then spoke well.
I have quite a few mutes for various brasses. I have reconstructed some, even made a horn mute tuneable by means of the cardboard center from a toilet roll. I tend to be with Tom Crown on the sharp edge promoting low range response. My newest commercial mutes are from Ion Balu. They are out of a combination of a synthetic interior covered by very thin veneer out of diverse beautiful woods. I like them and use them, but would have wanted the edge at the opening being sharper.
I have written on my latest homemade mute here, photo included:
viewtopic.php?p=309484#p309484
Actually I bought one more bowl two days ago, as the price was down to $16. The yellow one has the rope fixed for the 24" bells of my sousaphones. The new red one has the rope fixed for the 22" bell of the York Master BBb.
When taking the square rubber mutes off after warming up, my sound tends to be very clear with a crisp attack, as my ears have dictated me to counteract the darkening effect of the mute. When taking the Ion Balu mutes out, my sound is very round and full due to the same compensating effects.
The rubber mutes also work well on straight bells.
Klaus