It's true, most skimmers that LFS's carry are not up to the quality of what you can get for a similar price online.
I've build a few DIY skimmers and mods dozens. If you're planning to use a DIY skimmer on anything larger than about a 29 gallon tank I wouldn't recommend it, and even then it's cheaper just to buy a used skimmer.
The biggest issue in building a skimmer is having the right tools for the job. In order to get the best performance, some amount of acrylic bending and cutting is going to be required, and most people just don't have access to those type of tools. A quality skimmer is much more finely tuned and developed than most people think. The pump, venturi size and length, air-intake, reaction body width and height, neck width and height, as well as collection cup dimensions are all unique even though many skimmers look similar.
It much easier, and only slightly more expensive, to buy a high quality skimmer such as an Octopus, Euro-Reef, or ASM than it is to DIY a skimmer of equal quality. Trust me, I know from experience...