.065 kitespar. Or if you don't mind a little weight and want to get it down to no whip, a .050 kitespar with some strapping tape (the kind with kevlar fibers running along it) laid top to bottom lengthwise all the way around it and then spray adhesive and slide into a .065 kitespar tube. If you only use the .050 kitespar it will whip really badly. it does not hold up the weight of foam well at full length.
That sounds like a expensive core. Where do you get your Kitespar? I just got some of the fiberglass driveway markers to make a short sword. I might go get some more to add some strength to PVC.
I get mine at TAP plastics here in the Pacific Northwest. I would just search for Kite supply stores or google kitespar. Do not get the fiberglass rods, they are strong and heavy but they whip unless very short in length (like short weapons and shorter). There are some places online that you can get it delivered. Google is my best friend
The Fiberglass rod would but to get ones that would fill the PVC you are talking about adding a lot of weight. The tube gives better rigidity without the weight due to it being able to be thicker diameter without the weight of the thick diameter fiberglass rod. Kitespar is fiberglass/graphite tubes which will give you lighter weight. You could try the fiberglass tube in PVC. I would be interested in knowing how that worked. The PVC may hold up better against crushing than the Kitespar.
I was thinking about stacking the fiberglass driveway markers in a 2,1 pyramid and stagger the ends to add the length and glue them together and slip them into a 1/2 PVC with some kind of buffers. Guess I will have to check out the gaps and how to close up the ends.
Google Bandshoppe pole. It's about $10 for a six foot length, sliiiiightly lighter than PVC, roughly the same dimensions and infinitely more durable/rigid. I use it for two-handed weapons and it's nearly indestructible.