Not sure what you mean by "not airtight".
But airtight speakers don't perform to well.
If your concerned about foul weather I would research the brand & type you have installed.
If needed, there are Marine type speakers available. They're built to survive all the water and humidity exposure of boating. Although nothing is 100%.
I have Polk Audio Momo 5.25" [they are marine certified] in my batwing styled type fairing. Had to modify the opening a bit to accept them.
They have an airtight rubber surround that as per Polk "stands up to moisture, sunlight, and extreme temperatures while allowing long excursion linear motion and years of reliable use."
So happens, all my heavy wet weather riding occurred before I installed them. So I'm not able to provide real world defense of the product. But, they've gotten wet [not soaked] from the garden hose and have never failed to perform.
Oh yeah, did I mention that these Polk's sound great, bass, mids & highs, outstanding for a small coaxial speaker that is bi-ampable with inline crossovers, dual-layer voice coil that is melt resistant to 725* degrees.
Sound good at 80+ mph.
If you can get them to fit.
If you have larger speakers in there, look to Polk also.
Just my 2cents and that's all it could be worth.
http://www.polkaudio.com/caraudio/mm/index.php?s=mm5251I painted my grills to match the fairing so they'd be less conspicuous.