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For getting a FileSystemSyncAccessHandle, you need a FileSystemFileHandle first, which, in a Worker, where you want to work with the all-sync file methods, you generally don't have much use for, so you could "skip" it as below:

// In a `Worker`.
const root = await navigator.storage.getDirectory();
const accessHandle = await (await root.getFileHandle('fast', {create: true})).createSyncAccessHandle();

If, however, you decide at some point wanting to transfer the file from the OPFS over to the user-visible file system, you now need to postMessage() a FileSystemFileHandle in order to let the user run showSaveFilePicker() over on the main thread, where you can then write() the File you obtain via getFile().

Would it make sense to add a FileSystemSyncAccessHandle.fileHandle property, that would point at the associated FileSystemFileHandle? This would enable the following flow:

// In the `Worker`.
const root = await navigator.storage.getDirectory();
const accessHandle = await (await root.getFileHandle('fast', {create: true})).createSyncAccessHandle();
/* … */
self.postMessage('save', [accessHandle.fileHandle]);

// On the main thread after receiving the message with the `FileSystemFileHandle`.
/* … */
const saveHandle = await showSaveFilePicker();
const writable = await saveHandle.createWritable();
await writable.write(await fileHandle.getFile());
await writable.close();

(The alternative would be to just keep a reference around to the FileSystemFileHandle in the Worker until it's needed. It's really mostly a potential ergonomics improvement. Thoughts?)