This fan nacelle fits a 55mm duct but since the inlet is tapered there is some "fudge room".
this category seem to be the best fit this product and I only tooled up for it because I have others on the website ( 90mm, 70mm. )

I think I'll test it on an EDF striker so it will look like a bobcat.

The shape would look right on a scratch built A-10 , 737 or 757.
At any rate its better than simply strapping the fan unit to a plane.

Here is the link to the site.

Its not too hard to assemble since there are only three parts.
You start by marking the trim lines.
I would mold the lines into the part but that would surely make it hard to see. pencil marks are much easier.

Rough trim the nacelle sides with scissors.
Lay a sharpened pencil on its side and mark the side that does not have the step. molded in step.

The next trim line is the inlet ring.
This time place the pencil on a piece of 1/8 inch stock.
I used light ply here.

Trim some of the forward end of the nacelle halves away to avoid a poor fit of the inlet ring.
I took off 1/4 inch .
Bond the nacelle halves together at one seam only using model airplane cement.

install the inlet ring and spread the halves to check that the edge meets the step in the second half.
You should be very close.
A little block sanding should be all you need.

The rest is up to you.
In the past I have bonded the fan unit to the inlet ring then used silicone to bond the ring to the nacelle.

When I modify my striker I'll be back to show how I managed the nacelle.