PP is absolutely safe with NaOH. Rated A+ in resistancy actually. Source: Chemical Compatibility Database from Cole-Parmer

Also, when taking advice from soap makers who have been dealing with lye for a long time and very well documented, it is unanimous that glass is a bad choice due to shattering danger and PP is widely accepted.

Surface area for them to react? What? That's not how it works. You thoroughly mix the two layers allowing the non polar solvent to touch every part of the base mixture before allowing them to separate. The final amount of surface area in contact with each other means nothing.