I used a paint
scraper to get what was loose. Whoever did the carpet install on our Uhaul
was sloppy with the glue and had some outside of the carpet area. I used brake clean and a rag to get that off, I use brake clean for everything, it kills bees good too. Anyway I wouldn't worry about getting all the old glue off, just the stuff that scrapes off easy. For carpet I found both Lowes and Home Depot had a light weight
carpet made from plastic bottles, 6 X 8' or something like that. I put it up with contact cement applied with a 4" roller and disposable brush.