For the colonies in my garden I put the tray in 1 hour after treating then inspect daily so I can count and monitor more accurately. The 3 out apiaries have the trays in anyway in the bad weather- (there is a 1" space but it excludes the worst of the winds- but ventilation still good) I ensure clean inserts just after treatment then I look in 1 week time. if the weather gets milder then i will remove the trays about 2 weeks after treatment.
If you know where the cluster is, the board could be partly inserted with no affect on the required bottom ventilation.
A fully open floor is perfectly acceptable, but the bees will consume more stores. A fully closed floor is not acceptable for too long (assuming here there is no other ventilation, apart from the entrance slot) as that equates to a solid floor. Somewhere in between will be the optimum position, but the bees may not be clustering in the right place to check the mite fall!
Thinking 'outside the box' a bit - hives are different sizes (footprint), not all are even supplied with 100% mesh area, etc, etc.