Personally I have found the most difficult aspect of making a side entry trap is getting the springs right.
Commercial Larsen Trap springs are designed to close a top opener by pushing the door outwards where as with a Side Opening trap they need to push the doors inwards.
Unfortunately this means that the legs of the springs don’t quite orientate as well as you would like and don’t line up neatly against the trap ‘leg’.
To get around this I have fitted the spring to the side of the door using staples as normal— although you’ll notice it’s the other way round. You will see that in order to give the spring arm some degree of purchase I have place the short leg on top of the frame, secured it with a wire staple.