This idea might be crazy and there might be a reason it won't work that I haven't thought of. If so, revise it, or dump it completely.
Anyway, I've read where some use a PVC pipe with glued on end caps and a screw off lid to ship rods in. Sometimes these can be bent or crushed and cause damage to the rods inside. Also, by time you buy the parts, how expensive is it to make one of these tubes?
My idea is to use two 2x4's instead. They're cheap, under $6 for both. Get the lightest ones you can find since the obvious drawback will be weight. You can feel their weight and tell some are lighter than others sometimes. Saw them to the shortest length needed to reduce as much weight as possible.
Now put the rod on top of one laying flat and draw its outline on the surface. Draw it fairly close fitting but still loose around the rod. Now use a plunge router with a round nosed bit to hollow out the inside of the outline. You might get it deep enough on one board only, but if not then you'll need to make a mirror image of it on the other board.
Now put the rod inside it, wrapped up in some padding maybe, and tape the boards together, spiral wrapped around it all. Maybe even put a few screws along each edge to hold the two halves together, maybe to keep anyone from easily tampering with the rod before it gets to its destination.
This migh cost more to ship due to weight, but the making of it should cost less than the PVC version so that might offset the cost some. This also isn't what you'd do for an average rod to ship it but if you had a really expensive one that absolutely can't get broken, this could make a case strong enough to protect it against breakage from just about anything. A 2 piece rod could make for 1/2 the weight with just 2 short boards needed so cost is reduced to only one board and cut it in half to make both sides. The rod halves should still fit side by side on the board.
Maybe this will give somebody an idea to take a little further, or save an expensive rod from breakage, or an heirloom old rod. Don't know why I thought of it. I was half asleep at the time and it just came to me. Had to write it down before forgetting it so this was as good a place as any.