Egg crate is the way to go, no doubt about it. Help create a form for you to build on and with.
$10.00 - egg crate
$07.00 - fiberglass screen (stuff for windows/screen doors)
$12.00 - Concrete mix
$07.00 - Dye
$36.00 - Hardscape cost
I created two steps in the back of a 10 gallon tank. I realized a number of different flaws once I was finished and the concrete set :^) Learning experience for me. Information for you.
1.Make sure your filter doesn't produce too much flow. Mine does for this test model. For my future 40 breeder palud it will work well. Test it by coating your eggcrate with plastic trashbags in your tank. See if the flow is too much, or too little.
2.Always make it easy for you to access your filter again. I'll be cutting out an easily accessible compartment next time. Mine is sealed under 1/4 of an inch of concrete.
3.Water needs to enter the filter's intake again, if the holes to your rock wall aren't large enough you'll end up burning out your filter. Plan to have 4x the amount of flow at different levels. If one of the holes clogs up, you've got backups ready to take over. I clogged the mesh screen when I poured substrate into the water feature. Guess who is drilling holes tonight?
4.Depending on what type of plants you want to grow, it might be easier to make holes in the eggcrate and suspend pots/put in sealed tubes. Growing medium/substrate will wash away.
I'll be posting some photos. Tonight.
I used lava rock as accents, they blend into the concrete fairly well.