An enclosure might help a bit, but not a lot. What are you using for a plate, just spring steel? You will lose a small amount of heat to that. If it’s glass and steel, then you will lose more. The temperature sensor is pretty close to the actual heating element, and the heating coil doesn’t cover the entire bed, so there will be hotter areas and cooler areas—like an electric griddle if you’ve used one of those in a kitchen before.

Glass helps because it’s a good thermal conductor, but because it’s another layer to heat (and lose heat into) you need to bump the temperatures a bit, and it also comes with the caveat of weight, which isn’t ideal on a bed slinger.

Overall though, if you’re printing PLA and having bed adhesion issues at 68 degrees the temperature probably isn’t the issue.