I say, make one of the portal Giant Circular doors!
Maybe...
A falling brick wall door
A 'pick up the tile to get in' door
A small cabinet door
Tall doors
Glass door
I believe, you don't have to make JVS be doors.
Couldn't you make an engine, it would have an animation of the pistons moving 1 time for each side.
After you did that, you would make it loop in the events (oncontentstopped > contentstart)