How did you determine when it was at 80%? I just kept dissolving it until I couldn't dissolve any more and I must admit that I now have lots of crystals showing up in the mixture that I put into the bottle. Maybe I should add a little water and give it a good shake.
I prepared it by weight. Weigh out the amount of fructose (say, 20g) into a heat-resistant glass jar. Add distilled water (say, 5g = 5ccm = 5ml). Stopper the jar lightly. Place the jar in a small pot of water, such that the jar is immersed but not floating, and support the jar to prevent overturn. Verify that the lid of the jar fits loosely, not hermetical sealing. Heat the pot to boiling. Let it stand like this for a while. Most or all of the fructose will dissolve. Let it cool down. If there are still crystals, can add water dropwise.
It is possible to verify the final concentration by its refractive index, or alternatively its density, or its viscosity. The first one requires a refractometer. The second one - a sensitive balance (say, sensitivity of 0.01g). The third one - a very tall glass jar and tiny balls of known density, and a constant temperature ambient space. Neither option is easy under hobby conditions!
So, I would suggest that you simply do the following: heat your fructose solution as described above; bring to complete dissolution by adding DW dropwise. Let cool. If the result is a clear viscous liquid, use it. If it is turbid contains grains, filter through a coarse filter paper or even a nylon hose.
I have had a look at the earlier mounts I did with just nail polish. The polish seems to be shrinking and exposing the sample as I now have lots of voids which were not there when I first made the slide.
Nail polish contains volatile chemicals, of the acetone family. When it dries out too quickly, they evaporate, perhaps this is what happened to your sample. When mounting with nail polish, immediately seal the coverslip around with nail polish as well.