Hi Bob,
I'm coming to the end of my Juliet build now and initially started building my boiler until a cheap commercial one became available on a well known auction site.
As far as I am aware there is no specific plan for a tube construction boiler, because you don't need one. Neil is right, the original boiler build articles (not the oil fired type of course) published by LBSC in ME is all you need and I too am sure he covers the tube type construction briefly in those notes.
In truth, the tube type boiler is exactly the same as if it was made from sheet.
As long as the boiler tube is 3 1/2" O/D x 12 1/4" long in 16-gauge copper, you will have sufficient material to do the job. The firebox wrapper is cut 4 3/4" from one end and is split off centre by 3/16" (I think that's what the offset is?).
If you look at my Juliet build album, you can see what I've done with my boiler. It's a very forgiving beginners boiler really.
If you need a set of online build articles then you can get these off the John Tom website of just PM me and I'll send you the PDF.
Hope this helps!
Dan.