I mainly make cakes for family and friends. Does anyone have suggestions for a reusable cake board. I was going to have my husband cut a half 1/2 thick plastic cutting board, but I'm afraid it could eventually warp. 

1/2" plastic should not warp.  It probably could be reused many times before any problem would surface. 

I am a home hobbyist and make my own cake boards to fit into the plastic cake caddies...

I glue two pieces of 1/4 inch foamcore boards (the cheap kind from dollar general) together with a craft stick (non-toxic, acid free glue). Once the pieces are dried together, cut out the size/shape desired, sandpaper the edges smooth, then glue a piece of freezer paper, shiny side up, over the surface and around/under the sides (just like the foil covered boards are glued). Then I wrap the board with fancy foil/freezer paper and tape it down. Once the cake has been consumed, the wrap that was taped down gets pulled off and discarded, and the board is ready to be wrapped again. As long as no one uses power tools to cut the cake, the board will stay intact for many uses.

I have premade boards that fit my 12" round caddy (single tier cake), 14" round caddy (two tiered cakes), and quarter sheet cake with curved edges, with just enough room for a ribbon around the edges.

Once the premade board shows signs of being marred/warped/dented or the first layer of food safe covering (freezer wrap) has been compromised, the board is discarded.

If ever I need a stronger board, I will simply glue 3-4 pieces of foamcore together.

That's perfect, thank you. Great idea!

Why all the fuss? Just get a food-safe tray or a platter. Hygienic and washable.

Do you mean for between layers or the sturdy one under it all, that you carry it with?  If it is the very bottom one I do as Jedi suggested.  I have picked up a variety of glass cake plates from thrift stores.  I also have some round in different sizes and a rectangular glass microwave plate. 

Great ideas. Thank you.

My dollar store has some decent ones that work.

@Virago Great idea with the 1/4" dollar store foam core (cheaper than craft stores, even with a coupon).

I'm also a hobbyist and I've used all sorts of stuff.  Personally, I'll do just about anything to avoid having to deal with the hassle of getting a cake plate (or any other type of cake material) returned for re-use.  Since I'm already spending quite a bit to indulge my hobby by giving cakes away free, what's another $1 to $3.50?   Here are some of the things I use:

1/2" craft store foamcore wrapped with freezer wrap and/or Fanci Foil

12"x12" or 16"x16" ceramic floor tiles from Home Depot or Lowe's (I cover with fondant after cleaning the surface, and glue cardboard to the scratchy bottom.)   Tiles are especially nice for large gingerbread houses since there is zero wobble.   These are also multi-purpose because I can use them to weight a cake to make the filled cake "settle".  Ceramic tiles can also be used  to roll out fondant for flowers and decorations and letters. 

Inexpensive thrift store plates/platters, including microwave platters.  (For the microwave platters, I glue on some cardboard or foamcore pieces on the bottom so it sits level.)

I also purchase thrift store materials to make cake and cupcake stands.  I tell the recipient of the cake/cupcakes that the stand is part of the gift to keep after the goodies are gone.

For 8" or 10" cakes, I often use a "fancy" gold cake circle, then glue 2 more cheaper cake circles underneath that are a little bit smaller.  When I glue cardboards together, I alternate the direction of the corrugations to provide strength. 

* * * * * * * *

Here are links from my shutterfly account to show examples of what I mentioned above:

16"x16" ceramic floor tile:  https://christinascakes.shutterfly.com/pictures/311

Microwave plate:  https://christinascakes.shutterfly.com/pictures/387

Microwave plate (this is my very FIRST two tier and fondant cake ever):  https://christinascakes.shutterfly.com/pictures/102

Homemade Tiered Stands:  http://www.wilton.com/forums/messageview.cfm?catid=8&threadid=161628&startpage=1

Gold cake circle with 2 more, slightly smaller, cheaper cake circles glued underneath to provide strength (this 10"x5" cake was heavy!) and the 3 cardboards worked perfectly.

https://christinascakes.shutterfly.com/pictures/401

Since I can't edit, I'll try posting the link for "Homemade Tiered Stands--EASY!"  again.

http://www.wilton.com/forums/messageview.cfm?catid=8&threadid=161628&startpage=1

If the link doesn't work the second time, try this link (my inspiration):

Here's the link to WanderingChopsticks tutorial:
http://wanderingchopsticks.blogspot.com/2010/08/how-to-make-homemade-cake-stand.html

I've used round metal trays turned upside down for a long time. I purchased them for only a few (dollars) in the supermarket. They have an even round edge and I could use them many many times before an lines from cutting cake were visible.

Wow! Apti, that is incredibly helpful. Thank you everyone. So many awesome ideas. I can't wait to use them.

mommyofnine~~Thanks, but nearly as incredible as being a mommy of nine!