I added an endcap to Tara's steel PMC Comsol model and looked at the eigenmoedes for a 3-point contact. The lowest mode is a rolling mode at 2.0 kHz, followed by other modes at 3.0 kHz and 3.5 kHz. The first longitudinal stretching mode is at 16 kHz. The rectangular part of the spacer for this steel PMC has dimensions 5" x 2.6 " x 2" and a cavity length of 32 cm (first picture).

I also looked at a beefed up version of the spacer, with dimensions 10" x 4.6" x 2" and a cavity length of 78 cm (second picture). The lowest mode is again a rolling mode at 1.4 kHz, followed by other modes at 2.0 kHz, 2.2 kHz, and so on. The first longitudinal stretching mode is at 9.0 kHz. So it looks like if we want a longer cavity, we can almost double two of the spacer dimensions without shifting the resonances down significantly.

If we use a 10" x 4.6" x 2" spacer but go with a 4-mirror bow-tie design (third picture), we can get something closer to a 1.1 m cavity length. Comsol gives a lowest mode at 1.4 kHz, followed by modes at 2.4 kHz, 2.6 kHz, etc.