I’m thinking of solutions to building a thicker rod handle and I see a difference in making one with a density that pleases you and what skill level plus the necessary tools with which to achieve your goal.
I.E, the EVA handle in photos is tubular; do you have access to a lathe to shape the foam–and I’m not sure it can be sanded.
But if you merely want to increase the diameter and have a hard outer shell without a substantial weight increase, I think that I may be able to offer a solution.
1st, you will need th make a silicone mold of the finished handle. The material from which this is made is of no consequence so it can be anything from a broken rod to a shaped dowel or even a dowel wrapped in layers of cord, (which may result in a pleasantly textured finish.) The dowel will also allow for shaping the bore hole to slide over the existing rod and with a mold, if you are not pleased with the result, you can try again.
There are many You Tube video showing this and for inexpensive silicone you can use the large tubes sold for caulking kitchen sinks.
You could also wrap the Seiryu x handle with long strips of Ethafoam, That’s the white foam that is almost ubiquitous in shipping refrigerators, furniture and appliances. Amazon sells it as does Walmart but it’s so common that it can be easily found.
2nd, As for the material with which to fill the mold: Tear old foam boxes which formerly contained eggs or the bottom “pan” of poultry or meat from the grocery into small pieces. Do not use old styrofoam cups. Whizz these plus water in a kitchen blender until they are “fluff.” Spread the fluff on a pan to dry.
3rd, Pack molds with epoxy wetted fluff and let them harden for two or three days. For a smoother texture, lightly sand the handle surface and coat with epoxy.
This is a “additive” solution with the weight given primary consideration. For an “additive” process and which will be the easiest to do, Amazon sells a product called, “Delta-Lite Plus” fiberglass cast tape. This used by Orthopedic doctors to build a cast over broken bones. To use, the tape is wetted with water and wrapped around a limb, or in this use, your rod.