Unlike the conical Tornavoz, we're going for the cylindrical pipe-like port, whose behaviour is well-understood from loudspeaker design.
Card prototype 297mm x 75mm
Before we nail it onto the guitar (only joking!) it makes sense to try a prototype.
There is a God! The diameter of the Aria Contrabass Soundhole is 94mm. Its circumference is 296mm. A piece of A4 card is 297mm long.
A slice of A4 card, wrapped into a circle, is exactly the right size to slide into the soundhole and can be tacked onto the front of the guitar with some sticky tape. (Thank Heavens too, that the Aria is lacquered, not French Polished). We used Scotch Magic Tape ™, rather than Sellotape ™, as it is easy to remove and leaves no residue.
The temporary fixing for the prototype
The port is sited so that the top of it is reaches, but does not stick out of, the soundhole.
It's possible to slide the card in through the strings, so that different port-lengths ( card widths) can be tried in a matter of seconds.
A longer port-length gives a lower resonance, but once the cardboard port approaches the maximum possible length (the distance from the soundhole to the back of the guitar is about 110mm), there is no room between the end of the port and the back of the guitar for the port to "breathe". We found 75mm to be a good compromise, but the measurement is not critical - 70 or 80mm would be fine too.
Bird's eye view (exaggerated)
Our final version was made from acetate sheet. To improve its rigidity and make it naturally circular, the port was constructed from 4 concentric wrappings, with the joint of each layer taped to the centre of the next layer out, so that the joints were backed by solid acetate, as the exaggerated diagram shows. The innermost wrap can be butt-jointed, but each additional wrap has a small gap at the joint caused by the increasing circumference. This isn't a problem!
During construction, it's almost invisible!
This photo shows the innermost wrap which is butt-jointed with Magic Tape. It would be possible to glue the second wrap to the first if a more invisible fixing were desired.
Fitting the port
The completed port is still flexible enough to be installed in the guitar without removing all the strings!
Once installed, it's held in place with small pieces of Magic Tape - after all, it's hardly a big job to effect a running repair in the future should it be needed.
The finished Tornovoz-like port - invisible from the front
Hear the contrabass before and after the modification in the next section...