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Nice. I had to do something similar with my phone because I got tired of breaking the cheap plastic holsters. If you find that the tool is falling out being horizontal, you can always redesign the flap so it still opens vertically. something like this (not my project).A finger cut out at the bottom will let you push up the tool too for easy extraction.
Looks like the perfect project to try out some marquetry techniques on! If the inset rings begin to give you trouble you could fill them with clear resin or even do the whole top, and then sand it back smooth.
Cool project but I'd be careful about putting your handle to heavy use. Cast aluminum is on the brittle side given the nature of the material. You might end up with a stub in your hand and a 5 lb missile heading for who knows where. The reason ash and hickory handles have been used for centuries is due to the straight, solid grain pattern of the wood being free of defects that could cause the handle to have a weak spot.