12 V battery Terminal adapter

The battery terminals for virtually all modern cars are M6 (10mm a/f hex). The ones in your link look in proportion and I would think they are also M6. If the wing nut is larger, then you could always fit your tags to the pinch bolts or even better, use a longer bolt to take a second nut for the tag leaving the first one for the pinchbolt.

You just linked to one!

OP says posts... and links to something designed for posts.

I used a couple of small jubilee clips for the battery connections on the generator. Place tab of lug into gap between jubilee clip and post, tighten clip. Lug is formed to curve of post and the lead of the post is soft enough to grip the clip/lug very well despite the taper.

Of course not all posts are the same size, so the commonly available for cars clamps might not be suitable for the OPs posts.

No that is the 'type', but the studs are not M6 in spec its says they are 5/16" and 3/8"

I can find others on sale that are M8 studs .... but not M6.

I would happily cut off the crimps as I can buy M8 lugs easily - I just don't have a ratchet crimp tool for uninsulated terminals.

Where are the Aldi offers when you need one :-)

I think that'll be the post size, pos and neg posts are different. Those sizes would be about those of the posts on battery for the generator.

Drill out your lugs to M8? It's only an extra mm all round, maybe a tad less if you have the lugs snug on the M8 stud.

Drill the hole out, ffs.

Drill a hole into the battery post and tap it M6, then bolt the lug down,

Or turn the post down and use an M6 die to thread the post,

Yes I meant posts too. The clamps for posts are m6 and the sizes are standard and differ for + and -