Hi Kim...
What I do for delivering finished paintings might work for you... I get:
* 2 pieces of foam core cut to the dimensions of the painting
* 2 inch wide strips of foam core cut the length of each of the four sides
* a couple of ~2x6" pieces of foam core, and some scrap foam core
* a roll of clear packing tape (I bet you've got lots of that!)
Note: my local art supply store cuts the foam core for me, no charge
I make a 2 inch deep box with the foam core and packing tape that is the dimensions of the painting + a strip of foam core all the way around. (see below. . . )
The box opens like a book... with a 'bottom' and a lid. The 'bottom' has a patch of foam core fixed in it at the side each corner that leave just a little gap between the bottom of the box wide enough for the canvas to slide under... face up... this keeps the wet surface away from the lid when the box is shut. The lid has the 2x6 inch pieces as little flaps at the side to keep the 'sides' of the box from flaring near the open end... it helps if the top panel is a smidge longer than the bottom so that these flaps fit around the bottoms sides. . . (sounds more complicated than it is. . .
This box is light, fairly cheap, provides some protection to the painting and would probably still fit in your suit case... though what I usually do is make handles out of strips of packing tape and carry it like a briefcase.