Do you sand inside your hollow forms if so I thought that THIS may interest someone. You don't need to understand what he is saying as I think it speaks for itself
You can buy webrax mounted domes ready made quite cheaply - Silverline brand I think.
I found some in the local tool hire shop of all places.
Thought I saw some at The Toolpost early last year.
Edit: (to save having two posts in succession)
I also use extra long locking forceps to hold abrasives where the access is too small or too deep. Stainless steel, straight or curved, cheap & readily available - try your local fishing tackle shop.
Just remember that when using forceps you should hold the outside rather than insert your fingers in the holes of the handle. If - heaven forbid - the abrasive were to catch, fingers in holes become pretzels very quickly.