Filling gap between bottom rails and ali sides?

[jps]

Sorry to go on a slight tangent. Does the same not occur for all panels? I've just cut a rear bulkhead panel and, unless i do a fair bit of filing on the joints of the chassis tubes, it'll be very hard to get the panel to sit completely flush agaisnt the rails. I am imagining the same will be true when I come to offer up the floor panel as well...?