I recommend you to use one of the codes/standards in order to be on the safe side. For example you can calculate cylinder wall thickness according to ASME, the following formulas are taken from the pressure vessels, but they are good enough to give you a point:
for flat coverscover and cylinder respectively
"E" is a joint efficiency , if you use seamless part consider it 1, if not than 0.7 for circumferential stress. For longitudinal stress in your case ,I think, you should use 0.7
"S" is a permissible material stress= UTS/3.5
"P" is a pressure
"t" is a wall thickness
"C" is a factor of attachment of a flat cover, usually between 0.1 to 0.3
"d" inner diameter
"D" and "R" are outer diameter and radius
be consistent with proper units(SI,imperial etc.)!