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
and 935 ml of sterile distilled water, stir on a hot plate stirrer on low heat until the
Ammonium Sulfate is completely dissolved. Allow to cool, adjust the pH of the solution to
pH 5.2 using 1M H2SO4. Transfer to a screw top bottle and store either at room temperature
or refrigerated.
I would be tempted to filter the solution through 0.2 micron filter before storing.