A donor offered to fund a poll of this topic (that provides a link to download a PDF) in for the Niskanen Center. Rachel Bitecofer offered to administer it, and did so in cooperation with JMC Analytics, a Republican consultancy, to ensure legitimacy.
I was supposed to have access to this last night, with an understanding that it was embargoed until 6 AM this morning. Because of unexpected delays, it was not released on time, and around 7 I got an email, and Dr.Bitecofer began to a Twitter thread that discusses the survey, which you can see here:
The embedded Tweet gives the summary of the study.
As you can see, and as should be expected, the polling sample was 56% R to 34% D, and it was taken in key battleground state of PA, in 3 largely rural Congressional Districts. As you can see in the 2nd bullet, R voters have been persuaded by Trump NOT to vote by mail. A similar partisan split is shown on the question of reliance on USPS.
Before offering a bit more detail, let me note that the poll was in the field August 13-14, which is before people were finding out about the removal of the sorting machines from many processing centers.