Cargo Super Heavy Super Safe

I need something that mimics a stem

You do. You need something that actually clamps onto the steerer, because the pre-load-adjusting bolt is not secure on its own. It only needs to be gently tightened to get the correct pre-load, otherwise you're cramping the bearings (once you've tightened the stem bolts fully, you can then tighten the pre-load bolt for extra security if you like).

I have an old Kore BMX stem, which is very shallow, so it should fit under the top-cap. It has a nice wide flat face too, which might mke mounting a light or something easy. Yours for a fiver.