You could use slabs, but I would suggest a concrete base, more hygenic when hosing down.

hi,
Concrete is your best bet. Easily cleaned, easy maintanence. Make sure its a smooth finish. Then you can paint inside the kennel if you want and nothing will stick. I have 3 gsd
Regards roger

Most dog's i've asked like underfloor heating, with a solid bamboo floor, a couple of recliner's....a 46" plasma, and a place to store their slippers and cigars.

Best would be to use Shiplap board and either 2x2 or 3x2 for the frame. As a carpenter I wanted to do this myself for my 2 dogs but found it was cheaper to buy one off the Internet have it delivered and fitted than it was for me to buy materials to build one. Even with my trade discounts. That might be worth a look.