member 26b finally if desired an insulating material other than air can be disposed between the door parts during assembly as discussed above the e shaped track members 42 are not fixed to the frame 21 they are dimensioned for a relatively tight fit in the respective u shaped grooves 34 along the longitudinal sides 21b of the frame see fig1 but are not fixed to the frame by any other means the doors 26 28 are dimensioned so they extend partly into the e shaped track members 42 but not quite to the extreme end of the e shaped track members with the foregoing construction the e shaped track members 42 and the door members 26 28 can be initially assembled as a subunit the subunit can then be connected with the frame by inserting the e shaped track members into the u shaped grooves 34 in the frame 21 the cover unit is thus completely assembled and the doors can slide in the e shaped track members 42 in order to selectively open and close the opening 22 in the frame 21 the relative dimensions of the frame 21 the e shaped track members 42 and the doors 26 28 enables the doors and the e shaped track members 42 and the doors 26 28 to be retained in the frame the doors can be slid to virtually any position along the length of the frame selectively to cover the access opening 20 in the container or to allow access to the food in the container in order to disassemble the cover unit the doors 26 28 are both slid to one end of the frame i e see fig1.