Honestly few resins are truly non toxic. Always read the msds for materials you use, because those give you the best idea of the dangers involved. A lot of people believe that smoothcast is non toxic but the msds says otherwise. So just be aware of the safety measures needed. An option could be to work with the aqua resin and wall joint compound (spackle) or jesmonite as katsu recommended, and then to mold the piece and cast using smoothcast once you have the time and place to work in. That way the bulk of the work can be done indoors, with the final piece only taking an afternoon of slush casting. It's more expensive, but it would give you the strength of a poly/epoxy resin part, without the hassles of those materials.
Another option would be to build out of foam and use James Bruton's plasticising method he details on the rpf, along with a tutorial video on YouTube. His YouTube username is xrobots. (Website: xrobots.co.uk). This is another build it there finalize it later method, so it may allow you to at least get something built while on base. After plasticising the piece can be detailed with bondo and sanded normally, so it's not a bad option, being that it combines some of the benefits of foam building and bondo detailing.
I'm not experienced with anything other than polyester resin, bondo, and smoothcast, but from what I have read some of the alternatives can be a pain to work with. Hopefully you can find an option that works for you!
Good luck!
Drew