Is there a twin made that will physically fit in a GE 1 inch slot in a GE panel with no provisions for the 1/2 inch breakers? I saw an old 20/20 twin today in a 24/24 GE panel but couldn't see who made it, it looked kind of generic but it seemed to fit well.

My folks have an original GE panel full of twin breakers... some are GE, others are Cutler-Hammer, and Siemens.

Read pages 58 to 60 and 70, 71.
http://www.eatonelectrical.com/unsecure/cms1/TB00300001E.PDF

Best Plan, replace panel with roomy new unit.