Glenn's bases are aluminum but they are 7075 which is substantialy stronger than the normal 6061 that alot of makers use.
just a note here are some compairisons
416 SS that most gun parts are made from because its tough
Yield strength 40 ksi , hardness 155 Brinell
7075 that Glenn uses for his rings and bases
Yield strength 73 ksi , hardness 150 Brinell
6061 aluminum that most companies use for rings and bases
Yield strength 40 ksi , hardness 95 brinell
the next best light weight thing would be titanium or 7068 aluminum wich is almost as expensive as titanium.
You'll break the base screws before you break the base from Glenn.