I dun agree that canister cannot be use as DSB.
It depend on the design of your canister.
If yours is the same as the pic I drew below, then you can remove all the trays inside, so that the water will not disturb the DSB, and the filter wool is an additional layer to prevent stirring up of the DSB.
However, bec of the small capacity of the canister filter, the DSB may not be really effective. But consider yours is a 2ft tank, I think you can still try.
If your canister is Eheim 2213, 2215 type of series, then you cannot use, bec the water inlet is at the bottom of the canister.
for liquified sand bed, you need to have very packed sand inside, which the canister will not be able to provide that kind of pack environment.