You've not mentioned what you've tried so far, so I'll suggest two that I've been made aware of in forensics classes. Its useful not note that while these arn't completely foolproof, they do implement the one proper way to clear slack space in windows - using the defragmentation API to overwrite the slack space. If its a file you absolutely do not want found, encrypt it, and/or doublecheck to make sure its deleted.
sdelete is probably the gold standard for such software - its designed by sysinternals (part of microsoft these days)
sdelete -c should clean free space, or you can tell it to securely delete files or folders recursively. Its freeware, and pretty reliable. It does run on CLI, so if you're scared of the command line, you may want another option
If you need a GUI alternative, you can use eraser - its free and open source, has a fairly intuitive interface, and lets you know what files weren't deleted in the logs.
It will also let you schedule regular free space wipes, or other tasks, if it isn't a one off thing.
This is the main UI and the task scheduler window
Free space log. I have no clue what the file it can't wipe is, and I can't retrieve it the usual ways, so can't be anything important.