A 28mm brass compression olive fits with about a 1mm clearance. It would be possible to permanently fit it either by using something like epoxy, jbweld or similar and as a backup , then drill and pin the olive with a brass pin and flush off.
if you wanted a really tight fit cut the 28mm olive right through at one point only ,before applying the epoxy. Clamp until epoxy sets and then pin and cut off excess both ends. A light file and that's it. 15mm solid brass pins about £1 off ebay or a hardware shop, must be brass otherwise it will corrode.

the olive is that good a fit i just needed a bit of insulation tape pinched in one end of the spigot to hold it while i took the pic

IMG]http://i840.photobucket.com/albums/zz32 ... CF1341.jpg[/IMG]
use a normal size stainless hose clip about 9mm otherwise a larger clip will ride over the olive
Because the olive is to form a flange you want it at the end of the spigot, otherwise it would have been possible to use a 28mm nut on the spigot, then the olive and connect a 28mm female fitting to tighten the olive and that would be ok.
The ideal solution is a cv boot clip as its narrow and can be tightened up as it's designed to fix rubber to metal.
Its slim, very thin.
http://cgi.ebay.co.uk/CVJ-Boot-Clips-5- ... 1e58840e44
Just need a tool to tighten them up and problem solved
Then you would need to cut the brass compression nut off so that you are left just with the olive. The other problem doing it this way is the olive would be about 6mm up the spigot and not at the end.
see example picture

20mins work and your done.
Obviously clean and roughen the spigot before fitting.

The rad hose end will probably need heated (put in a cup of boiled water)to allow it to now stretch over the flange formed by the olive. Replace that cheap crappy hose clip with a better quality one.

i'll post a picture later with the olive slipped on the spigot as a picure tells a thousand words.
problem solved

cost ,
olive, peanuts
glue and pin, say fiver
i've got four spare olives if any one wants one or you'll get them in any plumbers merchant , b and q etc
Just a footnote. The olives sometimes come in two types. The very smooth one like the large photo and a more stepped one like the photo of the one fixed on a pipe. I'd prefer to go with the stepped olive rather than the smooth one, but it depends what's available. Since we're looking to make a "flange", with hindsight don't cut the olive fill the gap between the spigot and olive with the epoxy as it gives more resistance to "pull off" once pinned.
UPDATE
Olive fits fine, hose goes on with some saliva as lubricant. Problem! can't slip hose clip up hose as it's now tight at the back of the spigot against the engine casing.
A stainless cable tie works fine as it is very thin and the right width about 6mm. Getting it tight without slipping round the hose is a problem.
In the meantime i've got my brother to remove the olive (damaged my hand)and fit a stainless hose clip clamp 20mm wide 31-34mm size with a hex bolt for doing up. Sturdy bit of stuff.
For the olive to work it needs a really slim in width and slim in thickness hoseclip which can be fed tail first after the hose is slipped over the olive.
The roller bearing stainless cable ties don't have sufficient purchase to clamp up without slipping off the hose or damaging the roller calp on the tie.
The best bet would be these cv j clips if you get the tool to tighten them with. They are designed to clamp rubber boots to cv joints so are ideal, slim and narrow.

CVJ Boot Clips 5 Off 32mm dia' x 7mm wide Galvanised
i think these ones are better as they are stainless

£2.50 post free ebay
bay.co.uk/CV-BOOT-CLIP-19-40MM-STAINLESS-STEEL-BRAND-NEW-PACK-4-/370411256810?cmd=ViewItem&pt=UK_CarsParts_Vehicles_CarParts_SM&hash=item563e3b3bea
not sure if the 34mm size is better as the exhaust clamp ive used as a temp or permanent solution was a 31 to 34mm, a beast of a thing.

footnote
i see in the benelli tnt workshop manual which is available free online that the radiator filler cap is the same as the morini