I think putting the glass on the front (and also use a strengthened perspex I reckon, glass can break/shatter) so that the pressure of the water against the plastic would also provide a sealant.
I would also test the device with tissue paper in down to -15 (if that's the depth you are planning on taking it too, but again I am not sure whether it will hold, but then again, the only to find out is to test it!) So that if there are any leaks you will know becuse the tissue paper will be destroyed.
let me know how it goes, as I may end up making one!
Those plastic bags, I find, tend to crack around the seams as they dry out.
