See notes at end re what voltage is needed and why.
What country are you in. ACK DC20 supplies are available on ebay for around $US12 and some as low as about $US6. Even $12 is a very reasonable price as long as it is of adequate quality. There is a very good chance that these offerings are "after market" even though they may not say so.
The following is liable to work well if done properly BUT it's your camera and your reponsibility.
Any 2A power supply with Vout REGULATED between 6V and 8.4V, should be suitable, but 7 to 8V best.
Best not to use a supply capable of too much more than 2A.
Technically it should not be an issue but best not to give camera more than necessary ability to self immolate.
May need to connect an additional capacitor across the supply - say 2200 uF, 10V.
More may be needed but maybe none at all. Start small and work up.
A very few supplies will have problems charging up large external capacitor. Probaby not a problem. Ask if it happens.
The capacitor is to supply sudden large power surges,
Longer & notes:
All care. no responsibility.
A 6V to 8.4V range, 2A power supply with an optional 2000 uF connected across it should work.
One off custom battery plates are part of what what 3D printers are about, BUT the shell from a dead battery should work OK. Cunningly shaped good quality plywood would probably suffice or plastic sheet. Contacts will be annoying if you make your own battery plate , but brass and epoxy or superglue would help.
What Voltage and why?
Numerous sites state that it is a 7.4V, 2A power supply used for powering Canon ACK-DC20 G7 G9 EOS 350D & 400D cameras when they are to be used for extended periods in one location. It comes with a battery replacement dooddicky that plugs in where the battery usually goes and a cord is connected from there to the power supply. Without looking it up I'd be confident that these cameras used a 2 cell LiIon battery with a nominal 7.2 or 7.4V output.
The 7.4V is
More than you'd expect if it was intended for charging 1 x LiIon battery and
Not enough for directly charging 2 x LiIons and
"Just right" as a direct battery terminal feed voltage for a 2 x LiIon battery and
Connects to a dummy battery.
There is every indication that it is just a simple power supply and does not have specialist charger capabilities AND that it connects directly and only across the battery terminals.
You could with quite good confidence use an external 7.4V 2A power supply of almost any sort.
Better, as it replaces 2 x LiIon cells in series it could have a voltage of between 6V and 8.2V and be expected to work properly. Somewhat higher voltages are probably safe BUT better safe than sorry and stay inside the 2 x LiIon battery Voltage range. eg 9V is probably safe but not certainly so.
An off the shelf 2 Amp or better power supply settable in the above range, but preferably nearer the top end than bottom. ie
6V OK but higher better 7V Good 8V Good. 9V somewhat risky.
You MAY need a larger capacitor across the output than the power supply has. 2000 uF, 10V or better would be a good start. If the camera works OK with no cap, don't use one. If the camera behaves differently than usual under high power operations an added capacitor may help.