form the insert 40 additionally lock housings 54 are designed to be integrally cast into unitary members 56 which are adapted to be welded to the inside surfaces of the side bars 50 forming the insert 40 each corner member 48 has an l shaped portion 65 for receiving a squared end of a bar 52 each bar 52 is welded to the l shaped portion 65 as shown in fig6 each unitary member 56 has a flat surface 67 for abutting against the side bars 50 and for forming a welded connection with one of the bars 50 as shown in fig6 the unitary members 56 are positioned at a predetermined location on the bars 52 such that the top of the housing 54 will not interfere with a grate or cover not shown resting on the insert 40 both the lock housing 46 of the corner members 48 and the lock housing 54 of the unitary members 56 are adapted to receive the bolt 24 spring 26 and locking member 28 described above to form locking assemblies for locking the insert 40 to the frame 42 the housings 46 54 of the corner and unitary members 48 56 are essentially the same as the housing 22 previously described the housing 46 includes a stop surface 58 a recess 60 an opening 62 and a bore 64 similarly the housing 54 includes a stop surface 66 a recess 68 an opening 70 and a bore 72 though not illustrated a lock housing may also be integrally.