$$\require{cancel}$$
For a rectangular container the walls are parallel to each other and we can describe the higher frequency modes of the container as standing waves in three dimensions, much like the standing waves on a guitar string. As shown above, for a string the overtones are given by $$f_{n}=v/2((n/L)^{2})^{1/2}$$ where $$L$$ is the length of the string, $$v$$ is the speed of a wave on the string and $$n$$ is the mode number. Extending this to three dimensions, the equation for frequency modes inside a rectangular container with height $$H$$, length $$L$$ and width $$W$$ is $$f_{n,l,m}=v/2((n/L)^{2}+(l/H)^{2}+(m/W)^{2})^{1/2}$$. Now there are three different mode numbers since sound can travel in three directions. You can explore these modes in this box modes simulation Applet by Paul Falstad (notice you can grab and rotate the box to see different modes from different angles).