Re the knife: Depends a lot on the size container you are using. I have a Buck 522 "slimline" lockback in stainless that fits nicely into a small Altoids - it will stand on edge (kind of tilted) or lay flat in the bottom of the tin and is only about 1/8" thick. I have also carried a "micro knife" in PSKs from time to time - its a miniature version of a retractable utility knive, about 2.75" long and you can store spare blades in the handle - Lowes or Home Depot should have one of those.

Re the Tarp: When I did SERE training (way back in the 60s...) we made shelters from 3 panels of parachute and used small round pebbles inside the fabric and a loop of 550 paracord around the "lump" it made as tiedowns. Worked very well, and parachute cloth is very thin and somewhat fragile.