Hello,
what are the ways to extend the pot life of polyester resin (if possible)?

Some people use a MEKP/CHP blend as a summer blend for hot weather - to stretch out the gel time and not have it go off so fast/hot.  I think Norox MCP-75 is this... not my area of expertise.

Not sure if it would work for a polyester for open molding with standard promoters but Norox MEKP 925 H or similar catalysts have less peroxide and will give a longer gel time and lower viscosity before gel but still hit it hard for a complete cure.  I understand it works for infusion with VE but maybe the wrong match here so don't take my word for it!