Verifying source code with Remix on polyscan is a bit of an art. I tried the official ETHERSCAN Verification plugin for Remix, but the plugin does not seem to be configurable for other chains. I tried the regular imports which compile fine on remix
@openzeppelin... but the import is not supported. So then I tried the flattener plugin which did indeed do most of the leg work to flatten the solidity file, but the order was out of whack.