I see you were at ALA as well, I'd recognize those balconies anywhere. I was the Rufio running around half-drunk the whole time, haha.
Adding support to the blades of your current sword is going to help just the blade from bending, but without a support bridging the handle AND the blade, there will be a lot of stress right where the dowel ends under the slot. Furthermore, the lack of material in those slots may make it structurally weaker as well. If you try to split your current sword in half to put a rod inside, that just adds one more seam for the sword to break. For something as huge as this, building the support INTO the prop at the beginning is really important, rather than trying to add it after the prop is finished, so I'd just start again if you've got material and better tools like a hot knife.
For support, you're going to want something that runs through both the handle AND the blade. The circular materia slots prove a problem, however. If it were me, I'd actually run the dowel or pvc of the handle all the way up into the blade, through the materia slots, and then cover the open slots with halves of styrofoam balls or clear plastic ball ornaments.
Now, if you want to keep the slots open, I'd suggest you make your support two pronged instead. Use pvc pipe, and then attach a t joint at the hilt. A small length of pvc going forward and backward from the t joint, and then 2 l-joints to create two branches of pvc to go up the blade on the front and back edges. it should look like a huge tuning fork:
Glue all that pvc together and you should have a fairly sturdy skeleton. You could probably use wooden dowels of the same diameter with those pvc connectors for even more strength. Then, cut two blades out of insulation foam, half as thick as you want the final blade. Carve out a channel in each of your blade halves and sandwich your skeleton inside, gluing it all securely.