Putting an aperture stop directly in front of the lens isn't ideal but it may well have a net improvement because the slower f/ratio may have more net benefit than any aberrations introduced by the new stop location.
I think the longer the focal length, the less likely there is to have a problem.
I stop down my canon 135 f/2 to f/4 with an aperture in front and it works very well.
It's important that the hole is very smooth and round or it will show diffraction effects.
It's easy and inexpensive to try - so if it works and you see an improvement - then it works.
There are many ways to make the aperture. I cut a crude hole in a lens cap and then glued a smooth washer in place to provide a smooth round hole.
You could also 3D print something.
Edited by freestar8n, 19 February 2019 - 04:14 PM.