I installed a squirrel stove during the refitting of my boat. Don't know if this was an approved procedure, but was cheap and easy to do.
1. Cut 4" diameter (I think, can't remember the exact dimension) steel pipe to correct length and rake.
2. Cut hole in roof to suit, a little larger than pipe to allow for expansion/vibration.
3. Fabricated steel flange 1/2" thick to serve as an integral collar.
4. Welded flange to pipe in situ so as to achieve correct angle for roof profile.
5. Cut asbestolux sheet for underside of roof.
6. Lowered pipe/flange assembly through roof from above, sealed stove collar with asbestos rope.
7. Bolted flange to roof through the asbestolux.
8. Placed chimney on pipe, lit stove.
This installation proved to be problem free in the subsequent five years I owned her.