Looks to me its just the wrong bits. The Acorn Nut comes in different lengths, long, which you have, short, which you need. The actual lead only needs to be relatively thin, yours looks the size of a standards HT Lead, the disc is typical of a HT fitting.
Internet search for Lucas Acorn Nut will bring up a fair number of images, including one with a waterproofing boot.
For now, drill out a little bit of the centre of your existing nut so that the nut clears the centre contact and can be screwed further down into the cap. The lead just needs a homespun contact soldered to its end, which will fit in the new hole, typically the sort found in an inline fuse carrier. Screwing the nut down clamps it against the cap contact. A pop rivet crimped to the end of the lead would do as a short term fix. Crude but effective.
Swarfy.