If its ply lined, I just screw in some hooks to hang bucket, dry bag of scrims etc. Ratchet straps to secure barrels, back pack etc. Plastic storage boxes with lids from diy stores are handy too !
Sure the door cards are nice but I think people get quite obsessive about what is essentially a work van. If you really want to though check out aquaeous plastic fabrications he can build any tank or storage solutions you could want.