If you have borax and lye you can make your own Sodium Metaborate. But, if you are not adding acid to your stop baths, I believe Sodium Thio has a ph of 6.5 to 8.
It would be easier if it wasn't assumed we knew your formula and mentioned the name or posted the formula as well. What is your intended purpose of using the alkaline fix?
The essential ingredient of fixer is thiosulphate. Sulphite is a preservative and is mandatory if you want your fixer to last for more than a few minutes. Adjusting the pH is not vital. You probably don't need it to be significantly alkaline.
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