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I used to make my own hook tools before you could buy them. Here are a couple of links on how to make your own from masonry nails...

http://aroundthewoods.com/hooktool.shtml
https://www.westbaywoodturners.com/t.../hook_tool.htm

Some more on DIY... http://www.treecycler.org/hooks.htm

Michael Hosaluk showed me a HSS version that he had made for him but I haven't had a chance to try it myself as I rarely do end grain work myself nowadays. If I remember correctly, Michaels is HSS... https://woodturningtoolstore.com/pro...chael-hosaluk/

Other sources of ready made hook tools... https://www.hiltonhandcraft.com/Cata...ningTools.html

Hook like tools used for hollowing side grain can also be used for end grain work.

Before other options came along I used the original Shepid Loop from Woodcut for hollowing for awhile and that could be used for end grain work but was a bit tricky... https://www.woodcut-tools.com/arts-a...ter-p492085589

I get on much better with Woodcut's later open sided Pro-Forme cutter... https://www.woodcut-tools.com/arts-a...fts-p492088033

With the Pro-Forme, I much prefer the non-Flexi shaft arrangement because the standard shaft have a scraper head that allows it to rotate in the shaft in various positions for shear scraping inside hollow forms.

Here is another one if you are feeling extra adventurous... https://www.woodcut-tools.com/arts-a...ter-p492088059

And, then you get into the replaceable cup tip tools of which there are many. The only one I have used is the Rolly Munro, which are getting to be expensive, which in my case hasn't got much use since getting the Woodcut Pro-Forme...
https://www.packardwoodworks.com/tools-munr.html