I have a small bucket that fits inside my tank. It's maybe 4 litres. I drop a stone in the bottom of it, then the fish and fishstore water in, and then I float it all in my tank. periodically as I remember to, I squirt an ounce or so of tank water into the container using a turkey baster (AKA gigantic eye dropper). When the bucket gets nearly full, the temp is the same, and the pH should be very close.
(for the first few squirts, I only use about a half ounce of tank water, and I dribble it in slowly.)