If you have nothing else by way of an FAK, sterile pads and gaffer tape are your friends
I will try to write this in a constructive and polite way.
You should not have any packs on you body when sitting in a car, due to the airbags and seat belt design. If the airbag goes off when you have a chest pack on, the risk for fracture of your rib cage is quite high. Plus the seat belt, even if long enough, will sit wrongly, possibly sliding up un your neck, and either strangling you or crushing your windpipe.
Putting a weight on your chest is anatomically wrong. To counterbalance it, you need to activate your back muscles a little bit more than designed for. Large risk for (shortterm) damage.
Some decades go, in Sweden, it became very popular amongs the long bearded, sandal wearing male population, to carry their child in a Baby Bjorn. it was designed there, so the father could carry shopping bags and such ( free hands) and also 'feel close' to the child. So many guys got back problems, that the Government issued recommendations that guys should not use them.
Look around you worldwide. No culture carries any weight on the chest. Back, shoulders, hip, head yes. Chest - no.
The only chest carrying has been done by soldiers, with ammo pouches, plus grenades, and a long gun hanging off the neck/shoulders on a sling.
Modern systems have very flat pouches as close to the weight midline as possible.
But, with a backpack, you can get away with a couple of kilos on the chest. The weight can balance the back load a bit. BUT, the weight will put extra pressure on the shoulders and upper back.
as you surely know, ideally, a backpack should be packed so the heavy stuff is at the top, close to the center line, and maybe 80% of the weight hanging off the hips.
Yes, I am aware of the short distances the OP intends to use his chest pack on. It will be all good!
@Janne
What people call me, I do not care about. At this age (53) it is enough that I do what pleases myself.
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If you mean the chest rigg, it´s a mishmash, it´s not a ready-bought kit.Looks like a good bit of kit, what's it called?
I have built a sort of "survival" essentials chest rigg.