Here's a pic of one of my old setups. I unfortunately can't seem to find more detailed pics of the build.
I initially built it to hold a 20L on top and a 20H on the bottom, but I ended up modifying the bottom part to hold tubs for my snakes. Similar idea to bud's, but instead of cutting a notch in a 2X4 I used doubled up 1X3s to make lap joints. I agree with bud's suggestion - even though a 5g doesn't weigh much it's probably a good idea to have your horizontal pieces supported by wood (not just screws) where they attach to the verticals.