I've looked at websites, videos, etc. about various tuns (also called HLT or MLT?) and pretty much decided on the cylindrical water cooler type; probably 5 gallon as that's the size of the batches I'm brewing. My plan is to add a dip tube to the ball valve and mount the screen above the valve on stand-offs. That way there is a "smooth" bottom with no valves, pipes in the way when I stir. I may add a thermometer. What do you think?
Trust me, you do not want that much dead space in your mash tun. You'll risk bed compaction and a stuck sparge any time the liquid in the dead space falls below the bottom of the false bottom, which can happen if you attempt to drain the tun too quickly. Additionally, dead space is wasted space in a cooler-based mash/lauter tun.
I have been using beverage cooler-based mash tuns with false bottoms since 1993. Plumbing above the false bottom is the not a problem in practice because the mash should be floating.
Here's an internal shot of my current mash/lauter tun (my false bottom has 16% open space):
With that said, a false bottom is a waste of money if you are planning to use what Dave Miller refers to as "infusion rinsing" (a.k.a. batch sparging). Batch sparging and continuous sparging (a.k.a. fly sparging) are different animals with different crush requirements. Denny's setup allows for a much finer crush than a traditional false bottom-based design. Batch sparging requires a much finer crush than continuous sparging.