Your standard mold practice is fine for rotocasting, all you need is a good way to plug the pour hole and make sure the seam line stays closed. That said, that part seems a tad small for rotocasting, only in that you wont really be saving that much material once you've got a thick enough uniform wall built up, but that's not definitive by any means... maybe if you were making 100 of em... your long thin arm boxes could have trouble filling as well, unless you've got plans to solidify the arms.
You might look into fillers like urefil 15 to save on resin as an alternative. my .02