So I have been reading through the posts on desire for mithral tower shields and I decided to do the math to amuse myself. I plugged the three shields listed that are either wood or metal into a graphing calculator and ran a best fit line. .2x^2 - 1.2x + 7 if you care. Using this, I got a weight of about 358 pounds for the steel shield. This would be a steel shield 4 feet tall by 2.5 feet wide by 1 inch thick, based on the density of steel. Value for density taken from aqua-calc .com. Mithral is listed as half the weight of steel. Thus a mithral tower shield should weigh 179 pounds.
Or you could get the unique +1 ghost touch arrow deflecting mithral tower shield that somehow only weighs 22 pounds in Ultimate Equipment.
Of course, that being a custom item, it cannot be augmented further, but it is a heck of a lot lighter!
The things I am willing to do for a +2 permissable Dex bonus to AC.