Try Boyds Gunstocks www.boydsgunstocks.com I know they have a thumbhole stock for the ruger m77. If they dont have a munchkin version I bet they would cut one down. It's worth a shot.
Yo Ho Ho. Boyds phone number. The only number I have is for aJohn Bradford, Director of operations, Boyds Gunstock Industries,Inc.
It is 605 996-5011. Fax is 605 996-9878.
Well... that is all I could find, hope it helps ya'.