Before using this for samples that are precious, you should thoroughly test the solution that you make. The best way to do that would be a side-by-side with commercial RNALater.
The following recipe is from the patent. It always pays to read these.
In a beaker, combine 40 ml 0.5 M EDTA, 25 ml 1M Sodium Citrate, 700 gm Ammonium Sulfate