Mabuhay and Welcome to the board

Unfortunately dust will get into everywhere be it with a cover or not. As long as there a lot of traffic in the area of display, dust will be there faster than it would. Less traffic wont mean less dust at times. Regular vacumming - especially if you've got carpet (their dust collectors) in the room and I mean thorough vacuuming, not just willy nilly - so get a good one too, will help keep it in check. yeah Glass cabinets only seem to mask the dust issue, even the sealed once. Because the glass display is somehow clear you don't get to see the dust immediately but if you look closer - it's there. I have glass shelves but since it's tinted black I do see the dust accumulate, compared to the clear glass case (IKEA Detolf) I have, its the same when I run my finger on the shelf. but to the naked eye, it looked less due to it being clear glass, light refraction I guess. So for me, I just do regular vacuuming, and once in a while (like once every 3 months) I dust the shelves using a glass cleaner wet wiper. I don't dry dust coz that only pushes the dust and most likely flings it back in the air. try vacuuming as well as wet wiping (not water please)the shelves, once dry then placed back your figures. I takes me a week to finish - yup I don't do it all in one go as I get caught up playing with some of the figure while Im in the middle of cleaning hehehe. it's more fun that way and less like a chore. 
Well hope we see your collection photos when you've set it up. Cheers!
Wanted AM partner Vanguard, Myclones Dirge, G1 Victory Leo, e-hobby Dark scream ( the black version), e-hobby Magnificus
Parts- AM partner Basher-side guns, G1 Actionmaster Elite Windmill's blades[I][B]