I would not do this for several reasons, to put it simply heat transfer does not work the way people think it does. Commercial unglazed panels like heiocol sells is already well over 90% efficient at converting sunlight to heat under ideal conditions (no wind and daytime operating hours temperature within about 20 degrees F of the desired pool temperature). Putting the panels in a glazed box like you describe can in theory help IF the daytime air temperature is more than 20 degrees cooler than the desired swim temperature, in this case lets call that daytime air temperatures of below 65 degrees with a desired pool temperature of 85. The down side is you take a performance hit on the total amount of potential heat getting to the panels, common clear window glass is going to reflect about 11-15% of the light hitting it, plus it is going to absorb nearly all of the infra-red light and then reraditate half of that back out and half inside, so long story short. A glazed box will have about 50-70% of the heat gathering capacity of an unglazed panel, it will however benefit when temperatures are up to about 40 degrees different between the daytime air and the desired pool temperature. Of course this means the glazed array would need to be much larger than an unglazed to collect the same amount of heat, and at a 40 degree temperature difference the pool will be loosing much more heat, so the panels would need to be even bigger making such things impractical.
There is also the danger of melting the plastic panels in a glazed box in the heat of summer when there is no flow.
Ike
p.s. glazed solar boxes do have a place in domestic water heating where you are interested in peak temperature not maximum heat transfer