Why do you make a CRC32 calculator? Just for fun, I think.
BanchouCRC is a CRC32/SHA calculator GUI available on Linux and Windows (and if you get the source code, probably for macOS).
BanchouCRC has drag-and-drop support (limited yet), so you can drag your files to the program and make checksums and, of course, it supports multiple files.
This tools is useful in non-Linux environments like Windows, verify your downloaded files integrity and prevent malware or corrpution verifying their checksum.