We have a chassis that is 28 by 29 on top and on bottom. It is five inches tall and needs to be covered as it is only pipes right now. What kind of metal should we use?
This does not tell me anything, 28 does not tell me anything without context. Square inches? Square Centimeters? Some visual representation would be greatly appreciated, a photo or doodle would help wonders.
For a generic armor that can be acquired on the cheap, you can't go wrong with 4130 or 4140 chrome-moly steel. Its strong, easily weldable, can be hardened to further improve its strength, and is available at pretty much all steel supplies. As for thickness, well, as thick as you can make it and stay underweight.
AR400 is superior in terms of strength, and doesn't need to be hardened to obtain its maximum strength, and can be found at most steel suppliers(McMaster has it). Down side is that its much more expensive, is much harder to machine, and is harder to weld.
If you have money to burn, grade five titanium is ALMOST as good as steel, but is only about half the weight(Double the thickness to get more strength for the same weight). Downside is that its much harder to machine, very difficult to weld properly(TIG is pretty much mandatory), and is way more expensive per pound than steel.
Of course, you could always go the Beast
route and not even bother putting on armor. As long as your tubing is big enough, your opponent can't stick its weapon anywhere near your squishy bits, short of a tacitly placed pointy stick. If you do want to at least deter this sort of approach, a few thick sheets of UHMW plastic will keep them out, at least.