https://www.amazon.com/Positive-Negative-Voltage-Boost-Buck-Converter/dp/B07SJ4ZXYQ/ref=asc_df_B07SJ4ZXYQ/?tag=hyprod-20&linkCode=df0&hvadid=475741834053&hvpos=&hvnetw=g&hvrand=2655686133481039203&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9033519&hvtargid=pla-1032401992282&psc=1
Buck converter will give your -5v rail from the 12v rail.
Like I said in post 35, I have one installed in a Mage ST not a MSTE. Mage ST only has a 5v and 12v rails.
https://www.digikey.com/en/products/detail/mean-well-usa-inc/RQ-85B/7706154?s=N4IgTCBcDaIEoEUC0AOArAIRAXQL5A
Your right this would be a better choice. The other one you listed is to long to fit in a Mega STE