I would just take a lemon curd recipe and substitute limes for lemons.
This is the recipe I use (I think it came from Alton Brown??) I would just adjust the number of limes based on their size compared to the average lemon.
5 egg yolks
1 cup sugar
4 lemons, zested and juiced
1 stick butter, cut into pats and
chilled
1. Add enough water to a medium saucepan to come about 1-inch up the side. Bring to a simmer over medium-high heat.
2. Meanwhile, combine egg yolks and sugar in a medium-size metal bowl and whisk until smooth, about 1 minute. Measure citrus juice and if needed, add enough cold water to reach 1/3 cup. Add juice and zest to egg mixture and whisk smooth.
3. Once water reaches a simmer, reduce heat to low and place bowl on top of saucepan. (Bowl should be large enough to fit on top of saucepan without touching the water.) Whisk until thickened, approximately 8 minutes, or until mixture is light yellow and coats the back of a spoon.
4. Remove promptly from heat and stir in butter a piece at a time, allowing each addition to melt before adding the next. Remove to a clean container and cover by laying a layer of plastic wrap directly on the surface of the curd. Refrigerate for up to 2 weeks.
5. This recipe yields 1 pint.
Those are very different from key lime pie though... as key lime pie is eggs, sweetened condensed milk, and key lime juice.
Just do a 1/2 cup key lime juice (about 20 key limes), a can of sweetened condensed milk, and a cup of whipping cream. Mix the condensed milk and key lime juice.. set aside. Then whip the whipping cream and gently fold it in. You don't need the eggs. Put it in the 'fridge and it should firm up.
In key largo, they don't cook the key lime pies actually. The acidic nature of the key limes is supposed to kill any salmonella that is found in the eggs. When I'm making it for myself, I don't cook it either. If I'm making it for others, I obv. cook it.
Yes, it would be a little different from key lime pie. I posted it because OP didn't ask for a "key lime pie filling recipe." She asked for a "key lime filling recipe" for a cake, and mentioned that all she could find were pie filling recipes. Maybe I misunderstood what she wanted. 
Incidentally, I just returned from a trip through the keys where I came upon this great little place in Key West that was mobbed with people like ants on a lollipop.
Kermit's Key West Key Lime Shoppe
The pies were out of this world, and so simple to make..(recipe is on their site)...I also bought a bag of Key Lime icing mix that can be used to dust cakes or make key lime cream cheese frosting or key lime buttercream. They probably sell it on their site as well.
Yes, it would be a little different from key lime pie. I posted it because OP didn't ask for a "key lime pie filling recipe." She asked for a "key lime filling recipe" for a cake, and mentioned that all she could find were pie filling recipes. Maybe I misunderstood what she wanted.
Eeeeek I just read my post and it sounded harsh!! I was trying to write it quickly before my boss saw that I was on this site.. like usual! lol.
I didn't mean to sound as though yours was not the right recipe.. I should have phrased it differently. I meant to explain that if she wants it like a key lime pie.. this recipe is tasty.
I LOOVE lime curd and your recipe seems scrumptious
No, no...Don't worry! I didn't take it as harsh at all. I just wanted to clarify why I posted that recipe. That's the difficult thing with the internet. You can't hear a persons tone of voice when they post. 
All this talk has got me worked up for some pie now!
Ditto! I have to actually make one of these recipes posted above today! I have a wedding tasting TOMORROW..eek.. and they want graham cracker cake w/ key lime curd filling. I've done it at home (w/o the cooking the yolks) so now I've got to figure out which one of these I like best to serve to them. Yumm key lime!!!
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