Worn out truss rod nut help.

[L.A.H.]

I have this very old precision style no name bass which was my first bass ever. Over the years i've done various modifications often very amateurishly. Trying a post-modern (!) relic approach, then turning it in to a fretless and adding a jazz bass pickup to the bridge position. These were done a long time ago by me as a teenager with very little knowledge or care.

Recently i decided to give it a good round up make over. I really want to save this guitar as it has great emotional value to me and lots of memories to remind me of the past.
I removed the terrible varnish i put on the fertboard, gave it a good sanding to 9" radius and sprayed a decent layer of double component matt polyurethane (wood armour as they call it here). Did a complete rewiring with proper pots and the correct capacitor with a brand new swtichcraft jack. Removed almost all the post-modern scratches on the body, gave it a more traditional-relic look (this was my first try of relicking anything on purpose) and finally buff to spec, it finally had a shine after 9+ years =) Btw i must admit it sounded pretty solid even with those cheap pups and a curved neck.


anyway, my problem is the totally worn out truss rod that i hardly pull out of the neck. (its adjusted through the headstock)
This needs to be fixed in order to restore the playability.
Apparently the previous owner totally wrecked the allen tread in the truss rod nut which makes it impossible to tighten it further and leaves the neck with a decent bow.
I want to try and fix it before considering building or buying a new neck, i want to save this neck.
I need some advice how to fix it though ..
here is the picture of it (sorry for the blur i had to shoot freehanded)






At first i thought making those slots on it to be able to adjust with a straight screwdriver, but it proved useless as the screwdriver has to enter the truss rod channel (which is too deep in my opinion) with an angle and cant hold on properly to the slots, i might make them wider but im afraid of breaking the nut.

what im thinking is to cut the hex head off of a hex headed screw or bolt and weld it on to the worn out nut making it effectively longer thus possible/easier to adjust. Though im not sure if the welding can hold the tension when adjusting.
another option is to fill out the worn out cavity with welding material, reshape it to scale and cut bigger wider, better slots to adjust with a screwdriver.

i also tried to fit the bullet truss rod nut i had from stewmac but couldnt screw it on till the end because i couldnt find my imperial allen key set (which turned out to be at home for some reason so i will give it a go again hopefully this weekend) and didnt want to force it with the loosely fitting metric key.

But still, im not sure about the diameter or the thread on the turssrod itself either, the bullet nuts thread internal diameter measures 4.10mm and the original worn out nut measuers 4.30mm, i tried the worn out nut on one of the turss rods that i made with a 10-32NF bit and it fit nicely but had a tiny little bit of play... i have no idea what this means but it was not a smooth feeling trying to screw on the bullet nut on the bass neck, im afraid of screwing up the truss rod threads and im not sure if the 1/8 allen slot on the bullet rod can handle the pressure adjusting the bass neck...

i will be glad if you can help with the problem,
any ideas, opinions are appreciated