Can I build a DIY alternative to a Westcott Spiderlite TD5, or am I missing important features?

Not a darned thing. It's just five Edison sockets mounted on a base. The only catch is that you can't make a homemade version for photoflood or halogen use unless you're handy with a welder -- any wood product will turn into kindling over time when exposed to enough heat, and the kind of sheet metal that can easily be finessed with what the average handyman has in his toolkit won't be up to the task of supporting large fluorescents.

If you are using fluorescents (27s, 40s, 65s or 80s), then heat isn't a major issue. Ceramic sockets with a wiring pigtail run about $5 each (and will epoxy neatly into a 1-3/8" hole -- assuming the standard North American base. E-27 European sockets are a little larger). To avoid electrical dangers, use code-approved metal circuit boxes for the connections (and use proper wire nuts/Marr connectors or, if you can find one, a bus bar), and make sure you use a three-pronged arrangement with the ground wire hooked to the boxen. I'd also use heavy-duty light switches (the soft-touch switches glitch too frequently) in separate, but ganged boxes (you'll need three doubles or five singles). A couple of thicknesses of 3/4" MDF will be adequate for the socket substrate. You can box in the electricals to avoid that homemade look if you want to take it out for a fancy dinner somewhere (or have paying clients anywhere near it).

Just do it like you were wiring a circuit in your home and were waiting for the electrical inspector. It's not rocket science, but due care and diligence are always a good idea.