First of all, get yourselves some Dowty washers.

Available from Hitchcocks (Accessories/Engine/Oil seals) or other engineering suppliers.

These have a neoprene seal bonded in to them which seal a lot better than the original copper washers. Dowty washers seal with a lot less torque than copper washers, and in fact will seal with less torque than I consider safe to prevent the plug vibrating loose - much safer if they were wired on. I used to use Dowty washers at work to seal reducing valves onto compressed gas cylinders. They will seal against many atmospheres of pressure with not much more than hand tight torque.

Drill a small hole in the plug on a suitable flat which will hold the plug tight when it's wired.

Refit the plug and just nip it up. Drill a small hole in the engine fin to secure the rear plug and wire it on with a bit of malleable wire.
The best stuff to use is proper locking wire which resists breaking when twisted a lot.
WARNING pick up those offcuts of wire as they are sharp and can end up in tyres!
You'll see why you need to choose which flat to drill so that the wire stops the plug unscrewing.

And a small hole in the engine plate to secure the front one.
(yes, I drilled the wrong flat the first time on this one)

I know it looks a bit untidy but it really isn't that noticeable when viewed from above.

You can now put just enough torque on the plugs to seal the Dowty washers without fear of stripping the threads.

I'll do the oil tank drain plug as well when I next change the oil.