During long camping trips (sometimes as much as a month), I got tired of trying to "guesstimate" how much propane was left in the tank. The after-market gauge was useless -- it varied tremendously depending on the temperature and the altitude. Eventually, the gauge started to leak, so that was the end of that annoying device.
At home, I had been weighing the tank on my bathroom scale, but I refused to bring the scale on vacation. So it occurred to me that a simple spring scale might do the job (and would be very easy to store in my trailer). I went to Lowe's and evaluated their inventory of springs; I chose one about 12 inches long and 1 inch wide because it seemed beefy enough to lift a propane tank without deforming, yet not so beefy that it wouldn't even register the weight of an empty tank. It cost a little more than $5.
I attached S-hooks to the top and bottom of the spring. To make the handle, I took a piece of dowel about a foot long and cut a groove in it, so that the upper S-hook would stay in the middle of the handle. The dowel just slides out of S-hook for easy storage.
In order to calibrate the spring, I hooked it up to an empty milk crate and then added 7 pound bricks, one by one, until I reached a total of about 35 pounds (the weight of a completely-full propane tank). I then measured the length of the spring at each weight. Next, I marked a nylon strap with those measurements. The bottom of the spring indicates the weight.
Here is a picture of the handle, the spring, and the strap, next to a yardstick:
And here is a picture of the entire contraption, holding up an almost-empty propane tank:
The whole thing packs easily into an ordinary Ziploc bag.