Acrylic Thickness for a Bubble wall

Sorry this isn't necessarily related to fish but I know you guys build a lot of your own tanks using acrylic. I want to build a bubble wall like this http://www.oceanariums.com/shriners.jpg or this http://www.oceanariums.com/ymca.jpg. I think it would look pretty cool having a small one of each side of my built in wall tank. Are there any calculators for how think the acrylic would need to be since it is only a couple inches deep. there are also vertical chambers running the entire height of the piece. I think these things look really cool but are ridiculously expensive for an acrylic box an air pump and a light. I was also thinking about making a water column or tube since you can buy hollow acrylic tubes but again I'm not sure of the thickness needed for a given height and diameter. Has anybody built one or know of a thickness calculator other than cryo's since they don't let you input a thickness. Also any tips for making a wall unit to have the chambers in it as well?