They are 1cm deep x 27cm long.
Your alternative is a plank of wood the width of your entrance then shaved down so it slides into entrance and cut a tunnel entrance across the face of the wood. Same effect as hivegate.
Addendum. Just checked a couple of my empty nucs and there is a sufficient gap to get my fingers between the entrance and the bottom of the frames. I think it's important to have a gap so the bees can find their way into their exit which is now mid-floor. I know they quickly adapt to it.
Your alternative is a plank of wood the width of your entrance then shaved down so it slides into entrance and cut a tunnel entrance across the face of the wood. Same effect as hivegate.
Addendum. Just checked a couple of my empty nucs and there is a sufficient gap to get my fingers between the entrance and the bottom of the frames. I think it's important to have a gap so the bees can find their way into their exit which is now mid-floor. I know they quickly adapt to it.