Tilted Cake How-To

Bear with me as I try to describe what I'd like to try and do. :)

I had a friend ask me to make a cake for a Halloween party - she wanted it to be gross.  Then she threw in that it's also a birthday party for her husband's parents and wondered if I could work that into the design of the cake somehow.  Here's what I'm thinking of doing.  I'd like to make it look like a monster has possessed an ordinary birthday cake - so I'd have the top decorated in pretty generic "Happy Birthday Marj & Rick" with flowers and stuff, but then I want it to look like a monster has pried open the cake and there are snakes and worms and spiders crawling out of it.  There would be eyes recessed into the middle toward the back (I'm thinking of adding red LED lights in the eyes), some clawed hands coming out, etc. 

My question is - how do I balance the top layer at an angle?  How would I support it so it doesn't collapse?  It won't be a dramatic angle, but I'm afraid of it sliding or squishing the parts inside.

Any suggestions?