Yep,had this problem on all three TD5's i have had,so have others on here,there have been threads about it.The only dust i have seen get through is the very fine stuff.
i used a skinny bit of foam along the bottom lip of the airbox below the filter seal,with vacuum high melting point grease & it did the trick.The airbox now seals much better.

as others have said,the airbox & filter also have to be sitting perfectly,& the locating tabs not bent or damaged.

The airbox is a very poor design.

I recon that movement between the top & bottom of the airbox while driving may also cause some of the issues.

if yours has done almost 300k,with no issues,it can't be affecting the engine much.An oil test would also let you know how much dirt is exactly getting through.

Oh,no don't know of any aftermarket air boxes .