route out two grooves in the filler door frame make a step that will slide into the grooves when tilted a bit and still allow the door to close. so open the door step up, lean back over and close the door. With the step being removable gives same access you had before to the space.

Or attach legs to both sides of the filler door frame on the inside make a step that can rotate out 4" wide steel bar with wooden step bolted to it??? drill holes to fit the bar..slotted so it can be lifted out on out side when tilted up. Open door flip step down step up, lean over flip back in and close the door. Again the whole thing is removable with a tilting up of the bar.

OR

again add a 12"?? leg to one side of the inner door frame drill a hole in the top of it to take a dowel/post, build a step with a post on side so it can swing out or be pushed back in to one side. Use a conduit hanger or something similar to secure the top of the step dowel. so you a have step that rotates forward into the door opening and back in to the side. If the space above the step is long enough you could pull up on the step to release the bottom dowel point and tilt it out so it would be removable