I like to DIY as much as possible...
With that said, I've never tackled a skimmer. From my 'research' (if that is what you want to call it), it is very hard to achieve similar results to those that are made by reputable companies.
The easiest version appears to be the variety utilizing air pumps, a technology which none of the big manufacturers use any more.
Most of the cost when buying from a manufacturer/retailer can be traced to the pump. Unless you are going to make the pump, there probably isn't enough cost savings to justify the trouble of making one (or two, three, or however many it takes until you create a version that performs like you expect).
Tubing (whether PVC or acrylic) is surprisingly expensive.
I want to say that an old member of this club made a really good looking DIY skimmer. If you search through the threads you will probably find a reference to it. I can't recall ever seeing a build thread, just where he showed us pictures (I'm thinking of something from a couple years ago).
Good luck!