Just bought a Hashimoto HM3 SUT on fleabay

[mjw21a]

The SUTs made by Hashimoto (eg, the Live! MC-907 that use the HM7s) use shielding inside the box.

Yeah, we used mu metal shielding inside the box for my HM-7.

 

[avanti1960]

I'm not sure how seriously you should take Audio Technica's "100Ω minimum" recommendation. It would limit you to a maximum turns ratio of about 1:20, assuming a standard phonostage input impedance of 47k, but the cartridge has such a low output (0.12mV) that it could benefit from a slightly higher turns ratio.
The cartridge's coil resistance is 12 ohms and its inductance is only 8 microhenries, so I think the inductance is low enough to ignore for now. Here are a few figures for the load impedances and output voltages you would get with different turns ratios:
1:10 470 ohms, 1.17mV
1:15 209 ohms, 1.70mV
1:20 118 ohms, 2.18mV
1:25 75 ohms, 2.59mV
1:30 52 ohms, 2.93mV
1:40 29 ohms, 3.41mV
1:50 19 ohms, 3.66mV
1:60 13 ohms, 3.75mV

What's more important - the output signal level or the load impedance? Ultimately, what's really important is good sound quality, including a good signal-to-noise ratio. 1:60 would give you the best signal-to-noise ratio but I would be doubtful about the sound quality, not because of the low impedance seen by the cartridge but because it's so difficult to make a transformer with such a high turns ratio and good performance. I would expect a 1:20 transformer to give you much better performance than a 1:60 transformer, and the output voltage of 2.18mV should be enough to work with, particularly considering your phonostage has slightly higher gain than most mm phonostages.

I think you will always have to compromise with the ART7 because its output is so much lower than many other modern LOMCs but its coil impedance isn't particularly low. For reasonably hiss-free performance I think you would need a minimum turns ratio of 1:20. Is it worth going much higher than that? That would be a matter of speculation and you could only really know for sure by experimenting, but I wouldn't make the load impedance seen by the cartridge the most important criterion in your decision-making process.
Going from my experience I would say the range 1:20 to 1:30 is where you should be.

Can you please recommend a SUT for an AT ART9XA (.2mv, 12ohm internal) ?

 

[rothwellaudio]

Can you please recommend a SUT for an AT ART9XA (.2mv, 12ohm internal) ?

That cartridge has a fairly low source impedance, which is a good thing, and its output is isn't unduly low, so it's a pretty transformer-friendly cartridge. You could use a turns ratio as low as 1:10 and get enough signal for many mm phonostages to handle but I'd recommend a higher turns ratio. I'd say 1:20 is just about right. If you're looking for a recommendation for a specific make/model of SUT I suppose it's no surprise that I'd recommend one of mine - the MCL:
https://rothwellaudio.16.ekm.shop/mcl-120-mc-transformer-38-p.asp
If you're looking for a supplier in the USA, Brit Audio are stockists http://www.britaudio.com/