I have chosen idea number two because it is the best idea which is operated  off the ground. Although idea number three operates off the ground it is complicated and would be difficult to make. with ideas number one four and five the loop which would have to be used would have to be very large or the door or gate would have to be hollow so the catch could get a hold.
with idea number two the loop can be smaller and would be suitable for a solid or hollow door.
there are some changes i would make. first of all the backplate does not have to be in two parts it can be made as one (welded together) this would save bending and two screws. the backplate does not need to be bent so i would make it straight and put some fixing holes in it, this would save time when making.
i think the design will work and it would be easier to operate because it can be mounted on the door at any level.
what i will need next are sketches of my new design, a cutting list and a drawing with all the sizes.
if i have time i will go into my dad's school and make a model to see if the design is any good. the model will probably be made from wood because it is easier to work with when building models.