It happens often to me that I am on location and have to shoot with the flash and umbrella high in front of the model. Problem with that is that the boom holding it is right in front of my subject. When I have a tree on top I can hang the flash onto the tree, but that is not always the case.
Buying a sideways standing boom with sandbags and so on is not an option, because its too expensive and does not fit into my car.
Are there any tricks that I am not aware of?
When presented with this situation I've usually hired a VAL (Voice Activated Lightstand.)
VAL's are a self-propelling vocally directed support system, they come with built-in collision avoidance systems and fit well into most cars. They are also compatible with most types of light (within certain weight limits) and with the monopod boom suggestion presented in Mike Sowsun's answer.
While not in use supporting a boom they can be applied to other tasks and are amazingly versatile, I've used one as an equipment trolley and as a coffee maker.
They're generally easy to control and are available with support for local languages everywhere and tend to be widely available.
Probably the only downside is that you cannot buy one, you can sometimes get hold of one for free for a few hours but generally you do have to hire them (it's a legal thing) which can be costly for long jobs and heavy/repeated use.
You can put the flash on a tall monopod, and then use one hand on the camera, and one hand on the flash, like this guy: https://idigitaldarwin.wordpress.com/2012/05/20/aggressive-gear-pt2/,
If you find it too heavy, Use a tripod for the camera and two hands on the flash.
Buying a sideways standing boom with sandbags and so on is not an option, because its too expensive and does not fit into my car.
You can buy a 6' telescoping boom that fits a standard light stand for a couple dozen bucks. And you could surely DIY a solution using PVC pipe for half that. Either solution would easily fit in a Smart coupe.
