To cut a piece of batting that would fit into the suitcase perfectly, I used the pattern piece I had traced using the top lining piece of the suitcase. I used strong glue to glue this to the bottom on the suitcase (shown in the top picture above).
The piece of fabric for the bottom of the suitcase is cut about an inch or two larger on each side than the bottom of the case. Glue it into the bottom of the case, on top of the batting. I used very little glue over the batting but was generous with the glue on the sides of the suitcase. The raw edges of your fabric should go up the sides of the suitcase a bit. Don't worry, those raw edges will be covered by the lining pieces you add to the sides, which we will start next...

