That’s a great point, with my luck I’d bust through and I guess have to re-glass itYou can sand anything really. Start with a 80-100 grit until you get pretty close to your final dimension, then hit it with 600-1500 grit to sooth it out if you care. If it was me, I would use a dremel with a sanding drum on the end. Or even start with a carbide bit and hog it out then finish with sanding.
One of the important questions I would be asking is, how thick the fibreglass shell is... You really don't want to go through it if it's hollow in there...
Do the shoot2hunt guys have much experience with the AG Composites alpine hunter? Specifically the potential to bust through the top sanding too muchWrap the sandpaper around an appropriate diameter extended impact socket.