Don't use laminate for an exterior nestbox's floor... it is NOT wood, it is chipboard or mdf type stuff, and will start to sag and collapse Real Fast as moisture and gravity get to it. Scavenge some real wood or plywood scraps, seriously.
The best way to build an external nestbox is to, if at all possible, build it INSIDE the coop
External boxes are really impractical and an unnecessary complication (they will always be a weak point for predators and weather to get in) if you can POSSIBLY fit interior boxes into your coop. Yoiu can always have an exterior *access hatch* to interior boxes.
If your coop is super tiny, though, then yes it is sometimes necessary to have exterior boxes. In that case I would suggest you simply build yours better, out of more-appropriate materials and with no gaps anywhere, and supported *strongly* (screwing into the studs of a stud wall is good, with a cleat underneath to help support some weight if the box is large), and with rubberized flashing where the lid hinges to the wall. Make sure there is some mechanism to prevent rain that's tracking down the coop wall from getting behinc the flashing (there are various ways to achieve that). Weatherstrip the lid, and put on a seriously raccoon-proof latch.
But, it is a whole lot better to *avoid* exterior nestboxes if you can
good luck, have fun,
Pat
The best way to build an external nestbox is to, if at all possible, build it INSIDE the coop

If your coop is super tiny, though, then yes it is sometimes necessary to have exterior boxes. In that case I would suggest you simply build yours better, out of more-appropriate materials and with no gaps anywhere, and supported *strongly* (screwing into the studs of a stud wall is good, with a cleat underneath to help support some weight if the box is large), and with rubberized flashing where the lid hinges to the wall. Make sure there is some mechanism to prevent rain that's tracking down the coop wall from getting behinc the flashing (there are various ways to achieve that). Weatherstrip the lid, and put on a seriously raccoon-proof latch.
But, it is a whole lot better to *avoid* exterior nestboxes if you can
good luck, have fun,
Pat