I have run across a couple of techniques that are useful when building theatrical scenery to reduce seams. The first is leaving a slight space between the luaun sheets. The second is routing a v-groove along the seams. Both of these techniques allow the Bondo or putty to fill the whole- and this reduced the amount that it mounds over the seam.
A couple of ideas for mounting double sided flats:
Keyholes and pucks. Similar to the method for wall hangings – you would cut a keyhole into one side of the flat and on the corresponding flat attached a round piece of material (Sintra or PVC sheet, steel, etc.) with a spacer to the style with a bolt. Lift the piece into location and slide down to secure. Hold the pieces together – but does not include any compression or help with seams.
Z-clips work a little more like picture hangers. A picture hanger include a mounting plate and then a clip and is usually a little loose, but they can be very useful. Z-Clip use compression to snuggly fit too pieces together.
**For outwater check out item numbers alu111, alu211, aluhac22 and aluhac23.