I use a 350 mm x 150 mm double glazed unit and so I cut out a hole 335 mm x 135 mm this leaves me about 7-8 mm to make a rebate for the glass to hold onto. the rebate I do with the router. You can use single glass too but the double glazing cuts down heat loss and also doesn't steam up in the hatching stage when the humidity is high
(For some reason i lost the photos of the door so i'll upload some on the next one i make or make a drawing)
Once the rebate is made the door can then be covered with formica, both the sides and all arround the edges. If you use two diffeent colours of formica like i did, make sure you put the colours the right way round(oops when i realized it was too late) so now the glass is fitted from the inside rather than the outside as was intended. Stick the glass in place in the rebate using nuetral silicon as this gives off virtually no nasty vapours that may linger and damage any eggs. Once finished this can be put to one side so we can cover the rest of the main box.