Hi Cantputt,
I like the simplicity and straightforwardness of your TD.
Since you asked for suggestions, consider this "Outside The Box" suggestion.
Make it a "built pun" - literally, insulate the 'outside of the box' you've built as the body.
Doing such will not take away any interior space and it should be very straight-forward
to do. Check out the Foamie thread regarding the foam and glue and canvas and paint
various approaches. This should prove to be just what your TD is - a modest and functional
way to achieve your goal.
My approach to do it, would be to make a modest perimeter 'frame' on all the edges
of the body of 3/4 or 1&1/2 inch wood, depending on how much insulation you want.
To keep the door(s) and window(s) undisturbed, just make the 'frames' around them
beveled and set that beveled edge facing the door(s) and window(s) but set back a
half inch to and inch from their metal frames so that there is some room to attach
the fabric covering. [Or you could uninstall the door(s) and window(s) to add the perimeter
'frames' and then reinstall them.] Fill the open areas created by the perimeter 'frames'
with the corresponding thickness of foam sheeting [3/4 to 1&1/2 etc.]. Since the walls
are already painted, you'd need to use some kind of construction adhesive that would
adhere foam to painted wood. Make those as snug a fit as you can and fill any gaps between
the wood perimeter 'frames' and the foam sheeting with the canned foam and/or light
weight wall spackling [see the various techniques in the Foamie Section]. The outside
edges of the perimeter 'framing' should be rounded over by sanding or by using a router
rounding bit [about a quarter inch at least], though, for proper fabric application. Once
the exterior insulation and its 'framing' is all set and "fair" it is ready to go, then you can
apply the glue/paint and fabric and then the glue/paint again. You can use various kinds
and weights of canvas or a strong heavier cloth of some kind. It just needs to be able to
be saturated by the glue/paint so that it, combined with the glue/paint, will adhere to the
foam and perimeter 'framing' and it also thus forms a membrane that acts as the exterior
weatherproof skin.
The floor could either be done the same as the exterior walls and roof [Just more of a 'jigsaw
puzzle' to do.] or just lay some denser foam sheeting on top of it on the interior - with or without
extra framing support - your choice. Since foam will compress some with bearing point pressures,
I'd put some kind of plywood or such on top to help spread out such pressure since your floor
is used differently than most TDs - not as just a mattress base. Even those snap together foam
flooring pieces/"rugs" that you can get, could also work as a floor top surface.
Sorry for being so wordy, just trying to be as clear as I can in the description.
This is not the usual approach, but maybe it could do for you and save you totally reconstructing
the whole thing to add insulation.
Cheers,
Norm/mezmo