trehalose sugars are very good especially for drying and storing enzymes and proteins. Many pcr enzymes are very stable and ship at room temperature so if you are only storing for a few days then contact the company that sells the kit and you may find that the reagents store well enough for your use anyway

I would say maintain the optimized protocol and the temperature for storage and adding your sample in a sterilized environment or using ice or freeze dried procedures is enough to stabilize your mix though there are commercially available stabilizer kits. good luck

i agree with Paul. Enzymes are stabilised by amongst other things glycerol at between 10% and 50% but the former concentration is a standard component of PCR master mix which is why at room temp you could leave out for 1-2 days with absolutely no effect on activity; most of us have carried out this experiment by accidentally forgetting to put back in the freezer, used and found it works perfectly ok

If it is necessary to store at room temp deliberately for more than a few days - not ideal but there is a reason you ask the question - than adding 1/3 volume of glycerol to your master mix, taking the concentration closer to 50% will definitely help stabilise

Homercer keep in mind that this will dilute out your magnesium and also enzymes stored in glycerol above 20% have lower intrinsic activity: in other words they are more stable but less active. In consequence, just before use, you might want to dilute out the glycerol to its original concentration with a buffer that restores Your diluted mix to the original manufacturers conditions. check the composition of your master mix there fore but I would suggest it would consist of Tris pH 8.0 MgCl2 and also dNTPs, which also be restored to their original molarity