Yes. I am going to make a booster to fit on the port. I plan to make a in series booster like yours however it will be interesting to see if only one extra motor gives some meaningful performance increase.
I am a little concerned about drawing 9000w from property using the 4 vac motors and a heater.
I got a big roll of arctic 2.5mm three core from an electrical place and some heavy duty plugs and double heads . I made a 30m ext cord which i use every day , and a 70m for the odd long run .
When you set up your machine at the property , twin vac and heater can go to the one room .
You just need to subtely plug in your ext cord elsewhere as far away as possible from that room , if you are doing upstairs ... run the cord to upstairs , if u have easy access to the kitchen and not bothering anyone go there ... or some other room a distance away . That can be your booster power source .
You get used to doing it subtly without the watch of the owner , you dont want to wander round infront of the customer with a massive extension cord looking like you need to plug it in somewhere , might freak them out or they will tell you there is four sockets in the room you are working in .
there is heaps of electricity available for such a setup , i usually have air blowers on also . And if it just happens thats its too much hassle to source the extra circuit now and again , what odds you still have your twin vac setup .
Think of it like this also , a two man team using two heated portables in the one job is 12000w from the machines alone and it wouldnt be too unusual ?