I think that would be the easiest way.
You could also use styrene sheets, gluing them together with extra support on the inside. I have heard that Bondo will not adhere well to Styrene, so you would have to use something else, such as epoxy putty.
You can make curved objects by pytting the sheet in the oven in low heat for a few minutes, bending it by hand after you have taken it out and let it cool in that shape.
Styrene can be bent over an edge heated by a candle, but the heat could warp other parts out of shape where you don't want it to, so you will need flat metal objects to use to press it against a table or other objects. Burned parts need to be painted afterwards.
Joined edges of ABS/styrene can also be reinforced on the inside with ABS/styrene dissolved in acetone.It will take a couple of hours to dissolve, and a few more hours to cure. The surface should preferrably be mushed-up with acetone before-hand and the result will shrink a little bit, which could warp the piece. Best to do this on the inside.
Advanced technique:
To make the join more durable, put the acetone/ABS mush on fire (!) and press it together with pliers or other flat metal objects while it is burning. The mush will expand when heated, but pressing on it should counteract that, allowing it to properly fuse. However, you could need to have some experience doing this or you could mess up a whole lot. Of course, it will need to be filed, sanded and painted afterwards. Benefits are that it will join fast and the result could be as sturdy as a vac-formed piece.
