<p>I am trying to create a couple of holders for my ultrasonic cleaners. They are supposed to be used for parts that don't fit in the holder that came with the cleaners. I was wondering what material is best to use for this.</p>
<p>My initial thoughts are:</p>
<ul>
<li>Material should hold up to the cleaning solution, I have a wide range of them from degreaser, deruster, and so on. I would say PETG or PLA should be a safe bet as it reacts with almost nothing</li>
<li>Material should not have issues with warm (not hot) water, I'd say something along the lines of 60-80 °C. This already eliminates PLA, but I think PETG should still be OK-ish (I am aiming more towards 60 °C than 80 °C).</li>
</ul>
<p>Is there something I am missing? Does anyone have any input? I am anyway just going to do a few tests, but I assume starting with PETG is a good start.</p>

This is going to be hard. Even holding a vacuum is hard (I've tried it and not succeeded). I'm not sure what the mechanism of air molecules getting thru the print is - whether it's defects in inter-layer bonding, defects at seams, imperfect mating with the fitting, or even permeability of the plastic itself. It might not actually be existing flaws in the print, but rather the high pressure being a stronger force than the bonded layers can withstand, essentially ripping the layers apart from the weakest point until the pressure can discharge through the opening produced.
If using ABS, you might try an acetone bath followed by a long period of trying or use of vacuum chamber to quickly remove the solvent, if you can stand some possible part deformation. This would tend to fill any gaps. Coating with low-viscosity CA glue (Loctite 420 or equivalent) might be a better version of this approach, as the solvent will both attack the ABS and deliver fill material.
In principle PET (maybe also PETG, but PET is preferable anyway if you can get it) should be a suitable material for pressure vessels, as it's what's used for soda bottles at comparable pressure, but those are blown from a single piece, not fused together with seams.
At some point I will attempt this again, and will update my answer if I have any findings that contribute to your question.