I have a 6 and 8 year old and wanted them to be able to ride with me.
My car is a 77 with just provisions for lap belts.
I drilled through the parcel shelf though the metal, welded a 1/4” thick plate with a hole in it to the sheet metal in the engine bay and then mounted some 4-point Y harnesses from seatbelt planet:
https://www.seatbeltplanet.com/i-30498140-4-point-y-harness-lift-latch-buckle.html?ref=category:1389308
When I remove them I can plug the holes in the shelf with some vinyl covered plugs and it’ll be fine.
When tightened all the way my 8 year old is perfectly snug but my 6 year old still needs a booster. I’m using an inflatable one from BubbleBum as it’s narrow enough to fit the Porsche.
Then again, I’m only allowed to take them to school and back (4 blocks) until they can ride in the front seat according to mom [emoji2369]