You can sometimes bodge your way around this by putting a strip of thin soft plastic (like from a zip-lock bag) in to take up some of the space. I did it on one of my Waltlys (and maybe a Ripmo?) that had a similar issue with an oversized seat tube. It would still slip occasionally, but only about 1/20th as often. I used a strip ~20mm wide by 80mm deep, but cut it longer so you can hold it roughly in place while sliding the dropper back in and then just tear off the excess (don't try to cut it or you'll potentially score the metal of the dropper post outer and risk creating a stress fracture in the long-term). You might need to fiddle with the dimensions, and if you're unlucky it might not work at all - but worth a shot. I initially thought of using paper but was worried it'd trap water and cause corrosion. It's one area where cheap droppers are better than quality ones IMO, the cheapies often has a sandblasted exterior which holds in place better than the blingier smooth machined finish.