Depth is important in quenching. The tank should be at least 4" deeper than the longest blade you will do. For most folks, that is at least 12" deep. I like 16" as a minimum.
One reason why it needs the depth is so the blade and tongs can be FULLY quenched below the oil and moved up and down. Incomplete quench depth can slow hardening as well as cause flare-ups.
An old compressed gas cylinder (CO2, O2, Nitrogen, Argon, etc) with the top cut off works great. a 40 or 60cuft one is just right. You can saw the top off with a hack saw or have someone wth a metal bandsaw do it. A welding shop or some machine shops usually have one that is out of date laying around that they will give you. Other ideas are a piece of 4" to 6" pipe with a metal plate welded on the bottom as a foot. If you look at the local dump or a salvage yard, you can find all sorts of cylindrical objects. Old ammo cans, old fire extinguishers, ... the list is endless.