Just got my internet back so sorry for the delay.

The only problem with the knurled inserts is you can push them in and pull them back out but due to the extra length they maybe harder to lever out but a threaded insert would have the least likely hood of being pulled out and would be achieved with a shorter insert. A standard M12 bolt has a minor diameter slightly smaller than the 10.3 hole but it is a fine thread and I feel that there maybe too much wood displaced and may push the top face up. Maybe to help you could tap the hole out first which would make the bolt fit tightly or maybe file a flat on each side of the thread to allow it to cut its own thread. I would probably go for a 1/2" whitworth thread as they are a courser thread and again you could tap the hole or use 2 flats or maybe 1 flat. Problem would be using the M8 threads as it only leaves about just less than 1mm side wall thickness so the hole through the middle would have to be smack on .The minor diameter is a lot smaller than 10.3 but the major diameter is 0.7mm larger than the M12 . Could be made smaller by using a electric drill and a file on slow speed to reduce the outside or maybe even use a square file to open the threads up so instead of a 55 degree included angle you increase them to possibly 90 and that would decrease the outside diameter. Have done that out in the field a few times for worn or rusted bolts.

You can always try it with a piece of scrap first but in the past I have used similar home made items. Biggest problem is putting the hole through the middle without a lathe but not impossible as long as you have a drill press and a vice, easy as. Just make sure if you machine stainless it is at a slow speed otherwise you can risk hardening the item and ruining your tools. It is a lot harder than buying an insert but seeing as you are having a problem with inserts you could make exactly what you want and it would be quicker than the time in postage. And to me its a bit of fun and makes me think a little bit now that I haven't got the tools I used to have.