I recently read that a violin soundhole should not be too thin edged or tapered as that will reduce the resonance.I applied that idea to a classical guitar and it ran away with me.I made a bigger rim of 11mm ---.Better! Then I went to 18mm ---Better again.All with thin plastic and card.Next stop 65mm .Really good sound.Better high and low notes.
This is all attached with masking tape and it really works.I saw that Domingo Esteso made a guitar with air resonance about 75Hz.That`s D below bottom E.It works like a dream. One "failure" with a full bodied Spanish guitar seemed dull.Then I realised you cannot just measure and fit.You need to work out what the air resonance note will be.I trimmed off one centimetre of card at the bottom and Bingo it was spot on with really warm resonance on all strings.You can fit it from the outside as a small reduction of hole width will not reduce the air resonance nearly as much as the depth of the tornavoz.
The Tornavoz will reduce air resonance frequency,give great deep bass notes and makes the high notes very sweet.Once the sizes and sounds are worked out the card or plastic can be replaced with thin balsa wood.Maintenance is not a problem as balsa wood is easily replaced.That`s usually the main worry with fitting them. Well worth a try.
The air in the "tube " acts like the plug of air in the neck of a bottle. That gives it the ------------------Resonance.))))))))))))))))))))))).