My sincere apologies.
I was actually hoping you had found one that worked under such conditions.
I have a special "range box" from back in my Hunter Bench Rest days, where I could use a beam balance with decent precision, but it required a lot of very tedious set up and of course some sort of set up area with a roof at the very least.
Most of the ranges back then had rudimentary shelters, which might or might not keep the rain off you at best.
It involved opening the box from the side, using that side wall as a roof, sliding out a tray and setting up a scale, then putting up a wind barrier all around so you didn't have to wait for the scale to settle down and become reliable within a tenth of a grain.
After a few such menageries, I finally learned how to properly use the lever operated powder throw in a consistent manner so I could get it to drop charges consistently within a tenth of a grain of each other, with the powder of choice.
Nice powder measure. It could be adjusted in the field to be within two tenths of the desired median measured throw.
This was handy, as powders were quite temperature sensitive back then and our records indicated how much to increase or decrease our powder weights between strings for consistent velocities.
Whether it made a "real" difference or not ???????????? It made us feel like we were on top of the situation at the moment.