Maybe you could use one of those big, cast Aluminum clamping style motor mounts? They are common to ARF Trainers.
http://www.hangar-9.com/Products/Def...?ProdID=HAN40M
(just for illustration purposes, the one above is .40 sized...you probably want the bigger version )
Or...one of the adjustable type:
http://www.hangar-9.com/Products/Def...ProdID=HAN2033