the strongest way is to make a box like this but do not weld the lid on yet

see how my cage is really low





tack the main cage together & to the box,s.
once you have welded as much of the main cage as possible cut the tack welds holding the cage to the box lids remove the box lids that were tacked on .then lower cage into box,s this will give you the room to fully weld the main cage around the top of cage lift back up weld lids on then fully weld cage to box lids.