What about the doggie ramp?

 

The floor is Mopar part number 82209986. It lists for $278. I will sell it for $175 shipped anywhere in the lower 48 states. Payment can be made by Paypal or USPS money order.

 

I too was looking at making the area flat for my lab. I made a platform similar to harcharger77's but it was TALL. My lab did not like it and it's a pain to lift her up every time. She would ride on the other lumpy side anyway. I went to Harbor Freight and bought 4 moving blankets and folded them just so to fill in the low spot and then on one side I put her bed in. She is really happy and somehow makes it work for her. Total was about $30 and I can use the blankets for other things.