I would much sooner chase a supplier of marine hatches before barking up this tree. Have you seen the stuff that Bomar makes? Another company, Lewmar also does some extremely nice marine grade hatches....definately overkill for every single Expo rig out there.
Check it out, even at their prices I'm sure you'd save hundreds per hatch....
http://www.pompanette.com/bomar
Spence
Spence is absolutely right about the much higher quality of Bomar and Lewmar hatches and portlights, which are nicely made and come in different levels of "ruggedness," depending on whether your boat needs to withstand 10-foot or 60-foot waves. With the possible exception of a roof hatch, the marine hatches are likely overkill.
The bigger issue, though, is that these marine hatches and extremely expensive (sometimes four figures) in the sizes that most people would want for big cabin windows. Seitz windows, for all the shortcomings in getting them, will likely be about a third the price. Also consider that the price of the Seitz windows includes the shade/screen combination, whereas the shade/screen combination has to be purchased seperately for the Bomars and Lewmars at considerable additional cost.
The size selection is also quite limited. Most of the over-the-counder marine "windows" are much smaller than "look-at-the-deer-honey" camper windows.
However, if you want the And Pompanette is considered the "bargain" place to get this non-bargain equipment. (Also, Diamond SeaGlaze is another manufacturer worth checking.)