Suitable bolt with level-plug

Bolt cut and tapered, with spacer and location stub welded on the back of the head

Showing the effect of the spacer, the adapter is how held at the open end of the socket, allowing the user to press the adapter into the axle plug for maximum effect. Product placement for Halfords unintentional!

Adapter in use. This is on a spare axle in the garage. My drive is on a slope, so I usually back the car onto ramps until it is level, which gives me plenty of room to get underneath to check the level. It also means that the weight of the car is on its springs, so the axle is tucked up in front of the fuel tank, which is what limits the space available for socket drive and attachments. If I jacked the body up at the rear to be level, and let the axle hand on the check-straps, space might not be a problem, but that is a lot more fiddling about than simply driving it onto ramps.

Mangled level plug with 3/4" AF bolt welded into the square recess

Comparing axle level plug to gearbox level plug - the latter being significantly larger. Measuring the two shows no overlap in the diameter of the tapers i.e. the large end of the smaller plug is still less than the small end if the larger plug ... if you see what I mean.

Mangled axle plug modified to gain a 3/4" hex head, and hopefully no more struggles undoing it.