Of course Simon Tatham’s puzzle collection includes this one. He calls it “rectangle”. You can configure the game to leave spaces around the numbers, which increases the difficulty. There’s a web based version here:
https://www.chiark.greenend.org.uk/~sgtatham/puzzles/js/rect...
Stitch is much easier than the advanced levels of this one. I would know having spent quite a few hours on stitch trying to get to top 1000. Also stitch had introduced shikakus at some point.
Thanks for sharing this, these are great for those 10 minutes break I set for myself between one task and another. Recently someone also shared a "Guess my RGB" game which serves equally well for this purpose