Terry

Send Wrens an E-mail. Nowadays they make a series of standard thrust augmenters designed for different engine sizes and jet exhaust speeds, which you you can down to the length of your model. They are not very good about advertising their wares as they have quite a range. Each tube comes with its installation diagram. Very high speed/supersonic exhausts (such as the SuperSport) can genereate buffeting inside the tube and they have developed a slightly different and stronger design to cope with it.

If you send them the details of your model's requirement and the engine you plan to use - they are always very helpful.