My MVHR vents will be going through the gable end and the current terminal of choice has a very coarse mesh that may stop a horse, but not a horsefly, which means the horsefly will be stopped by the main filter within the MVHR unit.   I was thinking of making a couple of large boxes to go inside the thermal envelope, positioned just after the ducts enter the thermal envelope and just before the MVHR unit. Within the boxes I would have a fine filter (net curtain, stockings?) that is eas