Dimplex Opti-Myst Pro water vapor cassettes don't need a chimney or a vent because they don't produce any dangerous emissions. However, you will still need the fresh air venting, regardless if you are installing the cassette completely out in the open or if you completely enclose it with glass. The cassette itself needs that fresh air return/circulation to ensure the flames continue to flow upwards and not roll back down into the cassette which would damage the inner electronic boards.
To ensure you don’t get condensation if you are enclosing the cassette with glass, it would be a good idea to go with the Pro box or ensure the fresh air venting is completely on point.
You also want to make sure that the unit is serviceable so even if you glassed it in, you would be able to open one side and service the unit if need be.
If you go with the Pro-box, you get the built-in heater (optional) as well as the built-in fresh air venting and the glass. The backside of the box can always be knocked out to give double-sided.
Please see the air vent diagram below.