I think it would be easy to do. Are you looking into making a copy of the one they sell in stores? Or are you wanting to make your own design?
Also how much does the on in your lfs cost?
If you do a rectanglar box with min support since it is only a 12 gallon, and nice panels to go around it, making sort of a box.
Tools you will need:
Sander and sandpaper
Saw or some way to cut the wood
latex paint or stain and sealer
2x4's and wood panels?
If you don't count your time as cost I would estimate it would be around $50 +/- $10. for all the wood and paint.
I know there are construction threads, I just can't seem to find any. Will add to post if I do.
14 Gallon Tall Aquarium
Picture link Soon