You open up your fermenter each time. We spend so much time online, in books and podcasts cautioning brewers what not to do
and we ALSO preach on those same mediums the value of taking a hydro reading. If there really was a problem with opening our buckets to take a reading, do you think we would advocate it so much?????
As long as you sanitize, and are careful, you won't have a problem.
This is what I use, and it works with both buckets and carboys. I replaced the plastic one a year ago with an extra long stainless baster from a kitchen ware store and it is awesome. But the plastic one from any grocery store works fine.
Here's what I do....
1) With a spray bottle filled with starsan I spray the lid of my bucket, or the mouth of the carboy, including the bung. Then I spray my turkey baster inside and out with sanitize (or dunking it in a container of sanitizer).
2) Open fermenter.
3) Draw Sample
4) fill sample jar (usualy 2-3 turky baster draws
5)Spray bung or lid with sanitizer again
6) Close lid or bung
6) take reading
It is less than 30 seconds from the time the lid is removed until it is closed again.
Probably less if you have help. And unless a bird flies in your place and lets go with some poop, you should be okay.