The best i tutorial for a tank topper is here
http://www.egerbil.com/extensions.html
Im in the process of making two like that
Ive made my own toppers before from mesh and also old cages & fixed levels in them, of course you would have to make them suitable for hammies.
I don't honestly think that buying a tank topper is a total waste of money (even at the price they are!) since you get what you pay for so to speak - something like a good cage or tank will last years & also be something that your pet has to live in for the whole of its life. I think the advantage to making your own topper is that you can decide on the size & style of what you want - i also think its best to use good parts to make the topper as a flimsy one wont last two minutes