Steve,
The brass sleeve, for the thermostat's aluminum sensing tube, serves as a protective cover, however you don't need it though its nice to have. The depth that the aluminum sensing tube sits in the evpaporator's core is somewhat crucial; it must be deep enough to sense only the evaporator's core temperature. If you don't go deep enough or if you go below the evaporator then you can be sensing air temp rather than evaporator temp. You can measure from the top of the evap box to the center of the evap core to figure your depth.
I think you would probably find a thinner wall diameter brass tube at hobby store. If you have a wall that is too thick you won't be a quick response for the thermostat on off function. You can figure the length of the brass tube is typically about 4".
Don't cut or bend the aluminum tube. It is filled with refrigerant gas, so the gas expands and contracts to turn on and off the thermostat, you can read more about it here:
http://www.griffiths.com/achelp/achelp4.html
under the subject
The AC Temperature Switch - An example in detail with a typical 911 Porsche