Where can I find a new lid for a uniflow powder measure or what other lid will fit it??

Did you ask RCBS for spare parts? I'm sure they have them.....

I never liked the original lid when it went away I found a rubber stopper made out of black soft rubber like those used in a chem lab....A cap that fits inside the hopper stays in place and doesn't boounce off as the original did. Perhaps an old fashined rubber dain stopper?

First, never leave powder in your measure, capped or not. The chemicals in the powder will attack the plastic tube and vice versa, ruining both and leaving you with a sticky, putty-like goo. Horrible.

To keep dust out of the measure between uses, some sort of cap or cover is useful. I have an aluminum funnel I use to fill the measure and just by chance it has a lip on it that just fits over the tube if I put the funnel on upside down. Perfect.

Any small clean can (mushroom cans are perfect) would work, or even a sandwich baggie. Perhaps you have an old set of scope caps; the objective end might work as good or better.

Where can I find a new lid for a uniflow powder measure or what other lid will fit it??
I have been using a plastic cap off the end of a tube for mailing rolled up papers for 10 years since I could not find the original lid (too cheap to buy a new one). Any cover will work. Like Rocky says keep powder out of the measure unless in use. I use a tear off antistatic laundry sheet and place it in the measure when not in use and that keeps the static down.

Did you ask RCBS for spare parts? I'm sure they have them.....
If you contact RCBS, explain how the cap got lost e.g. moved to a new apartment, and offer to buy a new one, likely as not they will send you one for free. I offered to buy a primer catcher for my RCII since my dog took it for a dog bisquit and they just sent me a new one. RCBS has great customer service.

PVC pipe endcap might work? Plastic wrap and a rubber band will for sure (short term use). Replacement part is the best solution.

Cheezywan

I use the pan from my scale. Then, I know where to find it to do a check-weigh.

.