While I'm waiting for my armour to arrive I got itchy fingers so I modded a Doopy M38 scope to take real lenses for extra authenticity.
My cast was a bit off on the near sight so I cut it off with a fine blade and sanded it back to even again. I drilled a small pilot hole and then came in through the back with a 25mm spade bit. It made light work of the resin and left a nice neat hole. I cut the far sight too, cut a pilot hole and came though the back with a 14mm spade bit. I added a 14mm and 25mm lens which fit perfectly.
I'll paint the inside and glue it all back and show final pics but you get the idea. Pm me if you want to give it a try. I have lots of lenses I can supply. Total cutting time was an hour max.
Some pics of the stages so far...