I have not modified a Glock.I did spend 30 years making prototypes,models,molds,etc.I have a few suggestions.
IMO,its pretty risky to start removing material by eye with no way to monitor the remaining material thickness.
If you can lay hands on a dial indicator,a 1 in travel indicator would be ideal,
you can jury rig it into a wall thickness gage.
To go up in the mag well,the ideal might be a 1/4 in rod with a 5/16 ball end.I doubt you have one of those,but it doesn't matter.A long series 3/16 allen wrench or a bent rod,you just need a rigid "finger" to poke up in the mag well,with some feature that will act as a contact point inside the mag well.A smooth radiused end is ideal.Maybe a long carriage bolt screwed into a threaded hole,locked by a nut,and the head ground/modified a bit.
Then you need the means to fix the indicator(a magnetic base might be ideal)
so that the indicator contact point rests on the "finger" thingy I first described.Set the indicator to zero and you have a thickness gage.There are a hundred ways to set it up.Now you can measure remaining material thickness at any point.Its better to know its .061 than to see a flexible thin spot or worse,a hole!Ooopsy...
When modelmaking in plastic,one of my favorite tools was a simple scraper.I had 12 in pieces of feeler gage stock available.Looks lika a hacksaw blade with no teeth.Its hard,springy,and tends to have a good burr develop if you grind the end of it.I'd cut off maybe 4 inches of oh,.016 to .028 thickness stock with an abrasive cutoff wheel,then at a bench grinder I would just grind whatever shape I wanted across the end.Convex or concave radius,straight,angle,depending on what I was shaping.The grinder will leave a burr on the backside.That burr is your cutter.I might dress it abit with a fine india stone,but don't remove all of it.Lay that on your workpiece,flex the feeler gagea bit for some cutter pressure,and drag it on your workpiece.It will cut fine shavings.What is nice,your workpiece will be smooth.No "fuzzies" like a file or dry sand paper,and no filemarks.
When you sand plastic,use the black wet or dry paper,and use it wet.Much better results,and no fuzz.A red rubber eraser makes a nice conforming sanding block to wrap paper around.
IMO,grab the grip in a the fashion you intend,and consider two things.What feels like a high spot? a pressure point that is annoying?
And,taking a natural grip,where is the point of aim?A laser cartridge would help here.Scrape to "sight in" your point and shoot.
I suggest its like fitting a piece of steel to a piece of wood,lower offending high spots...but know what you are removing and why.Take off as little material as possible.IMO,sculpting for a fashion designer "look" would not be productive.