@Sandor I see you may have a F45 CC… 1990 Freedom 45 (The listing is gone, so can’t see the details.)
I recently took ASA103 and got into a pretty detailed discussion about heaving to with a self tacking jib, then did a bit of research.
Does your F45 have a track with a car for the sheet, or the 2 blocks where the sheet makes a triangle? (If anyone has better terms for these, please let me know.)
With the track, apparently some cars have a pin you can set to keep it to windward. Other people talk about fixing something in track to block the car’s movement, as @newt2u said above.
Without the track, it seems the only answer is to tie it to windward somehow, and I haven’t figured this out yet. I was told a snatch block to the weather rail as @SunnyIsleMark isn’t a good idea because it’s not secure enough. However, I have a feeling the person didn’t consider how small these jibs are. Still, I’ve considered – but have not yet tried – using the same approach with a line running between the forward and midships cleats. This might be foolish.