Remembering using Decon ultrasonic cleaning fluids at work, I looked it up – if you need find what's in a substance use its "Safety Data Sheet". Decon 90 is a solution of potassium hydroxide and detergents; so though not a chemist I would think ordinary laundry detergent with washing-soda (not as hazardous as potassium hydroxide) would work as it is not so dissimilar to the proprietory fluid.
Note though, that alkalis will attack aluminium.
I am though, very puzzled by this thing about putting the work-pieces in a hard plastic or glass container within the tank. It does not seem to have come from the tank manufacturers; but I accept some may advise or suggest specific types of intermediate container for their tanks and for specific types of fluids – those are important variables!
Looking at it from an acoustics perspective – and the parts I was cleaning at work were for ultrasonic transducers – I would think some of what has been suggested on this thread would diminish the efficiency of the ultrasonic action, or even negate it, turning the process into one of simple washing.
Judging by the properties of the materials, a polythene bag or beaker might not present much impedance as the speed of sound through polyethylene is not too far different from that probable in a rich detergent solution. Glass though, is likely simply to bounce most of the sound back off into the water. If you then fill the jar or beaker with something other than water, you add still another mis-match to the system.
Ultrasonic cleaning is predicated on placing the work-piece, with the worst muck removed first, directly in a suitable solvent or detergent solution in the tank itself; and the manufacturer has gone to considerable trouble to couple the transducers to the tank to minimise the losses due to the two surfaces between transducer and fluid. So why add more, and worse, un-matched, barrier surfaces?
(This matter of acoustic coupling explains the gel used between transducer and skin in medical ultrasound – it is a matching layer to help as much of the signals' energy across the boundary as possible.)
Even if your intermediate container and the liquid within that are reasonably well matched to the surrounding fluid, so the work actually is being cleaned ultrasonically, the smaller container will cramp the rinsing action and keep the dirt around the work-piece.
Essentially we seem to have here suggestions to buy an expensive bit of equipment then not let it work at its best. It may still work, but is it working as intended or merely as a solvent-tank? If the latter, you might just as well wash the items in the kitchen sink.
The baths we had at work came with stainless-steel mesh work-holding baskets like those in chip-fryers, but you could dangle smaller work-pieces on wire from some point above.