I tried the hot iron thing once or twice and never got it to work very well. There were spots where I couldn't get the two surfaces to bond no matter how much heat and pressure I applied. I'm not sure what I did wrong.
When you think about it, vacuum veneering is just a method of applying pressure to the veneered surface until the glue dries. A vacuum bag does that extremely well, but there are other methods that can work well. Each panel of a well-built speaker box should be quite rigid. If you have two such boxes -- a pair of speakers -- you can clamp them against one another and veneer one face of each box at a time. Because the boxes are rigid, clamping the boxes at the corners should apply good pressure over the surface of the mating panels. Use a glue like Unibond that has a fairly long open time. Let the glue dry, remove the clamps, trim the veneer flush, rotate the boxes and veneer the next two faces. Not very fast, and not very high tech, but it works OK. It does, however, require that the mating sides each be flat. If you have sanded the boxes and rolled over the edges, you won't get good results from this method of veneering.