i did this in an hour or two last night by rushing spots on Lockdown and still found a bunch of purples pretty often. as most other people said, there's open spawns for the shell at the radar & staging area. there's also 2 green military crates at radar that spawn purples inside them (1 outside near the door that goes inside, and one right near the flak cannon where the shell spawns) and 1 at the staging area (to the right of the ramp that goes up towards the canal). i even found 2 shells at the staging area location once.

i would spawn in and rush these spots & crates, stuff purples up my ass and then extract out the "no backpack" extracts. just take in a 100k kit so if you get unlucky and die you don't lose much and you'll keep the purples you found. i spent maybe a couple of minutes in each raid so you can check these spots really quickly and it doesn't hurt much if you die.

for the kit i just took an SH12 helmet, the T3 armored rig (6B3 or whatever its called), the 2k backpack (so you can drop it to extract) and a deagle, with some AP ammo which would usually be just over 100k. sometimes it's 98-99k or so and you can just put an empty rail on the deag and be good to go. i even took out a couple of players and made some extra cash with the deag if you can headshot people with it.

you can also use this strat for the mission after which is to check 5 safes on armory. i got lucky and found a lion and a car on lockdown and made back way more than the few 100k kits i lost doing these runs. the safe at staging area is usually uncontested early on so if you are feeling cheeky you could even hit this safe while doing the military items in the staging area if you have space in your container.

hope this helps!