Chuck,
>The rubber mat technique fails for the same reason, slight
>irregularties in the mirror, mounting plate, mat, or heat distribution
>cause uneven compression of the mat. Using a rubber mat is better
>than just leaning the mirror against a "flat" plate, but not nearly as
>good as a flotation cell. A technique like this that relies on
>compression needs a lot of compression to have an even distribution of
>force, but this causes widely different compressions (and thus focus
>shifts and unevenness) with different tilts.
You're right, of course, but I found that at least the gasket material (keep in mind, this is not just convention rubber mat) worked just fine in my 13". Perhaps it was just dumb luck. To overcome what you described as uneveness at different angles, I found it necessary to add three 1"wide x 2"long x 1/4"thick pieces of aluminum along the bottom of the mirror mount. Also lined with felt, these prevented the mirror from shifting in the L brackets when tilted near the horizon. (Because of the mirror's lighter weight, these were unnecessary when I made a similar mount for an 8".)
Cheers, Phil Harrington