I guess I don't get too fancy, I just cover mine with freezer paper. I use only masonite boards and get most of them back. I live in a small town. I have heard of people using contact paper, plastic tablecloths and things like that. Hope this helps you.
I cover masonite boards with the same materials I use to cover any boards -- gift wrap topped with clear contact paper; or colored/patterned contact paper. If I want to cover the board with fondant, I'll top the masonite with a piece of 1/2-inch foamboard so there's room for a ribbon border. And I'll only do THAT if I need the masonite for extra support. Otherwise, I'll just use the foamboard by itself.
I want to buy what looks the best and have the buttercream absorb through.
Did you mean NOT have the buttercream absorb through?
I'm glad you mentioned the cake sizes. One piece of 3/16" masonite may not be enough support for the full sheet cake. If it were me, I'd add a layer of 1/2-inch foamboard or, at the very least, double up on the masonite. If your masonite is thicker than 3/16", then I must know where you got it! 
I've been wondering what masonite is & where can you get it? I use 1/2" foamcore
I use masonite for all my heavy cakes. I use the 1/4 inch thickness. I love them, there great. They absolutely work on sheet cakes. I've never had any problems with them. I cover mine with wraping paper and then cover it with celophane.
Amy
Yes, I meant NOT have the buttercream absorb through. (been one of those days)
Dandelion, you can buy masonite boards on-line. It's the material that they make clip boards from and is very sturdy.
DianeM- The sheet cake is going to be a single layer. do you think that this will be too much weight? If so were can I buy foamboard?
*My DH says that this is my hell week right now. I'm being pulled in 10 different directions right now and can hardly think or see straight.
I get my foamcore board at Hobby Lobby. Michaels has it too. If you don't have one of those an art store should have it. If you do have a HL the coupon for the week is 40% off one reg. price item. I think the 1/2" was $5 before coupon.
I LOVE foamcore... but for sheetcakes I always do covered plywood.. It's cheap.. and I can charge a small fee for it if they don't want to get it back to me (NEVER bends or warps... ) To date I have about 8 of them and have never had a problem getting them back. The only thing is I don't have any rounds... I just use the drums for rounds.
Do any of you know if Home Depot or Lowes would cut ROUNDS out of plywood?
I went to both Lowes and Home Depot and neither would cut them. I cover them the same way I cover the cardboard ones mainly fancy foil.
The hardware stores might not cut rounds, but it's possible to get a drill bit that's a 'cutting' bit, especially for things like this. I think the one we found was about $8. Just draw the circle on the plywood or masonite, and use the drill to cut it out. There are also special arms you can buy to help make perfect circles. I'm planning on getting a set-up like that, when I can, because I really hate having to go to the cake store every time I need a drum. 
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