After adding all of the polystyrene rods I have sanded all of the added small parts in the places of rods connection areas to achieve soft shapes. After that I have wrapped these junctions with one light layer of fiberglass. After the curing of the epoxy resin, I have softly sanded the whole surface.
After finishing the frame, you can melt the polystyrene out of the frame by using acetone, so you can save about 90 g of the weight. I didn't do so, as I think the core helps to dampen the vibrations and shocks.