Hi Bill!
Daniel Fernandez has a recipe for cloning gel : https://scienceinhydroponics.com/2017/07/making-your-own-diy-plant-rooting-gel.html
Part A : 3g IBA + 0.6g KOH in 500ml of distilled water.
Part B : Carbopol 950 in 500ml of distilled water.
Can you recommend a preservative for the gel?
You can use potassium sorbate, but as far as I know none is needed or used in most commercial formulas, though a little won't hurt and be beneficial to the rooting clone unless you are using trichoderma or mycorrhizals.