Not looking to build one, just curious.  Seems like all the silencers out there favor weight over that last, incremental margin of silence.

What if they didn't?  How much longer?  How much fatter?

Imagine a 55 gallon drum.

As above, also I believe in theory should be legal to build sans stamp as long as it doesn't attach to firearm.  Have seen similar things built, tractor tires filled with lots of foam and plexiglass on both ends, hole for shooting through.

I've gotten this confirmed, by way of a feel good letter, from the tech branch. Shouldn't be difficult for anyone else to do the same.
Yet to reach reasonable reduction levels with anything moderately sized. Best results were with a 24", corrugated, drain pipe, about 10'L.
Tried one elaborate design, in the 5' range. Didn't feel it really did much.

As for direct suppression:
The best can I've used to date, on subsonic 300blk, is a mid-2000's YHM 7.62 Phantom. Not metered, but to the ear.
It's about 8"L x 1-5/8"Dia and has 4, slant wall, baffles.

Yep, surprisingly good sound with subs through the Phantom.  Much lower tone than some other cans.
I love my Ti QD .30.  It may not meter the quietest, but it's one of the best sounding (non integral) cans with subs.

Your question has no answer, there are too many variables.

Most commercial cans excel in one area, but fall short on another.

To answer your question, you need to define all the parameters including: cartridge, action, barrel length, bullet, temperature, humidity, etc.

A silencer that works well on a 30-06 bolt gun is not necessarily going to be the best option on a 300blk AR with subs.

Fatter and longer can equal quieter, but it can also mean loss of accuracy if the barrel it's attached to isn't strong enough to hold the weight or it screws up the natural harmonics and causes excess barrel whip.

30 cal, light and short enough to be used on a rifle.

Maybe you didn't understand

.30 cal? 300blk, 30-30, .308, 30-06, etc. - There are so many .30 cal cartridges no one can or even one baffle type will be the "best"

rifle? Is there a .30 cal pistol? What type of rifle? How long of a barrel? how will it be shot (prone, bench, off-hand, etc.)?

Where in the world is this shooting to be done? What's the weather like? Altitude?

You haven't even come close to being able to give you any useful information.

If subsonic I'd say a long large diameter tube with as many modified k-baffles you could fit would be quietest.
with supers?? hard to say. alot changes with supersonic builds. you have to size the suppressor to the powder charge for best results. like stated earlier a 30 carbine would need alot less volume vs a 300 norma magnum. example the 30 carbine with a 7" tube may sound great with a small blast chamber and 6-7 60' cones. where as the 300 norma mag may sound quieter(not than the 30 carbine) with 3-4 baffles and a larger blast chamber and more distance between baffles in that same 7" tube than 6-7 with a small blast chamber.

From what I've been able to find reading as much as I could find around the net, a good rule of thumb is, subsonics more baffles, supersonic gotta balance gas expansion space and baffles but doesn't seem to be any magic number.. or maybe there is but we'll have to hope HPA passes to play with them enough to find out.

FWIW, my 5.56 can is 9" with 1 blast baffle and 7 regular baffles.. I'm happy with the sound but I don't have anything to compare it against... on a 11.5" barrel its quieter then my 19" 22lr...