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You can actually take two steel drums and weld them together to make sure you have enough space for the animal to go in and the trap to close. The problem with a lion is the tail and it is thick enough to hold the gate open if they decide to back out. Usually when it drops their reflex is to jump forward and spin and then you have them.
Usually biologist and wildlife departments use galvinzed culvert material to build these traps and has them set up with a trailer tongue attachment and wheels to transport the animal from the catch site right in the trap. This type of cougar trapping is illegal for private individuals in the US but where it can be utilized it is often wise to build a second back compartment with a see thru material where a small kid goat can be placed and the lion can only see it by coming in thru the trap gate. He cannot harm the animal because of the barrier. The goat will call repeatedly and if you know you have a lion close by he will be lured by the distress calls.
Be sure to check you trap daily and ofcourse feed and water the attraction animal. Although you do not worry too much about human smells around these traps do try to keep it to a minimum. Cats hunt basically by sight and sound but use latex gloves when handling or setting the trap.
Good luck!
