If my memory serves me, what some have done is by a stainless steel socket head screw of the appropriate diameter, cut threads into the outside of the screw head, and cut off the screw shaft. I use anti-seize on the OEM plug.
I bought some stainless steel Allen head plugs a while back. I believe I have some extras. If you send me a pm with your address I can send you some. Might be a few days. I don't get back home till Friday.
I would suggest taking meathead up on his offer if you can't find them locally.
If you use a proper JIS screwdriver, the slot won’t booger up nearly as easy.
I wasn't under them impression that a JIS screwdriver makes a difference for flathead-drive screws. Phillips-style yes, like for the brake fluid reservoir screws, but a the interface between a flathead screwdriver and screw isn't that exacting.

Just use the biggest blade that easily fits the slot.