At this time I do not have large cooling racks. I have orders coming up that I will need to cool 16", 14" and 12x18 cakes. I fear if I cool on a cake board, that the cake would stick and be unable to slide the cake onto the top of another. I am amazed by all the help that comes from cakecentral and I know that someone will be able to help me.
Is there a way you can borrow a larger cooling rack?
I don't know if there's a way to fasten 2 of the smaller ones together so that they would hold. I'm drawing a blank, but I hope someone can help you more than I can. 
But I agree...they would stick to cake boards when you flip them over.
You can place the cake on a cakeboard if you line it with parchment first. If you have a gas cooktop, take the grates off and arrange them on your counter or table and place the cakeboard on those. Move the whole thing periodically so the cake is over a cool part of the counter.
If you have an outdoor BBQ you have a very large cooling rack already. Take off the grill part, scrape it clean from grease and junk(best way is to start the grill up and let the heat do all the work) Wash it. Invert two loaf pans and place the grill on top. Bingo you now have a very large cooling rack.
I cooled mine in the pans I baked them in the last time I did my 16" cakes. I put them in the fridge and left them overninght. They came out without any problem. You do have to grease and use parchment paper. I was a little hesitant about doing this but Collette Peters suggests it in her books, I assumed she knew what she was talking about and of course she did.
I never turn any of my cakes out onto a cooling rack. I place on large plastic sheets with a board under it. Then I freeze them til they are hard enough to handle.
If you use the 40% off coupon at Micheal's, you can buy the large cooling rack for around $5.
When I have a coupon and nothing else I need or want, I buy extra cooling racks. Hubby says I'm nuts, but I have 4 or 5 now! 
I never use a cooling rack anymore for any of my cakes. I leave them in the pan for 10 minutes, and then wrap them with Saran Wrap and leave them on the table until they're cooled.
I had the same problem years ago. Believe it or not I used the shelf out of my refrigerator! It was strong and easy to use. When I bought a new refrigerator (the old one no longer worked) I kept all the shelves! Still have them!
Oh for goodness sakes, so did I! Mine wouldn't lay flat, so I just turned 4 coffee cups upside down under each corner of the rack. This helped the cooling also because it was elevated off of the counter so the steam didn't build up under the cake, causing a puddle of moisture to build up.
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