Hi all!
I asked that question on DP Review, and I was redirected here because are analog specialists.

I need to expose 4x5 and 8x10 color neg film sheets in medium-ish-gray-like density (Kodak Portra 160). I do have the backs & dark slides, but no camera. I do not want any image on there; just plain and consistent medium-ish gray all over. I need this to be repeatable, and it need to be about the same from 4x5 to 8x10.

Based on the suggestions I got on DP review, I'll add this:
-A lab will be processing these, not me.
-I know the result isn't going to be gray when looking at it, I know about and I'm ok with the orange cast of the neg base
-I may not be the most technical person, but I did learn & work in analog photography before converting to digital
-I do need to do this with a color neg, not a B&W
-I'm not looking to make a filter of any kind
-I work with the break-down processes of the negative material
-The specific density of the neg isn't that important; I'll be able to judge from the tests strips

I am unsure how to proceed, and due to the cost of these materials now a days, I thought I'd ask first!

Here are the options right now:
1- I was thinking backs facing a large softbox in a dark room and:
A- incident metering at the place of the film back. Then, take off dark slide, flash, close dark slide. Setting on light meter: Iso 160 (for an iso 160 neg film), aperture to 1, speed
to 5 sec (so it catches the whole flash duration). Then reduce light of 1 stop. Or put light meter to iso 80.
B- measured at the place of the back on a grey 18% card in reflective, same light meter settings.

2- Expose with my B&W enlarger with a 80A cc filter to correct for the 2900K light bulb. I'd take a sheet and make time-strips of .5 sec or .2 sec to decide on correct exposure.

3- Rent an 4x5 and a 8x10 view camera and point it on a white wall illuminated with flash, work in reflective and correct for bellow extension.

Any thoughts on what could work and what would not? Other, simpler suggestions?
Thanks in advance,
PS: English isn't my first language, so please forgive my mistakes.