Good question... prowling around the outside of a dumpster or going over a porto-john with a tape measure is generally something most people raise their eyebrows at. If you're lucky, you live in or near a metropolis like Los Angeles where such things aren't any more unusual then most other things you see going on in public.
If you live in a more "reserved" part of the country, it could be a little more difficult. Especially if local law enforcement hasn't turned totally apathetic towards such things.
Good rule of thumb... don't act "weird". If you want or need to measure something like a dumpster, walk on up, whip out your measuring device and notepad, and get on with it. Acting suspiciously tends to make other suspicious of your actions. Acting normal makes what you do "seem" normal, and no one will pay any attention.
Be cognizant of where and when you are doing your research. Deciding you need to measure that dumpster down the alley in the middle of the night is going to cause trouble. If you need to measure a porto-john, find a construction rental company and go measure one in their yard versus loitering around one at the faire or something.
Lot's of common things can be "guesstimated". Walk past a dumpster, note that the top edge comes up even with the bottom of your shirt pocket, then once comfortably in front of your computron, search Google images to your hearts content. If you know one dimension, you can generally figure most of the others out, at least close enough to get the eventual piece to look right.
Hope my ramblings give you or others some ideas.
Great idea to have a resource of some sort for people to be able to post "research data"!
Proud son of Rose and Wes
"Women uh... women sense my power and they seek the life essence." General Jack D. Ripper