clearance in front of sub

Hey guys, I'm planning an install in my '79 Mini and need a bit of advice in terms of clearances in front of the sub. I've attached a Sketchup pic of what I'm doing. (with some bits removed for clarity) Basically its 2 10" subs in sealed enclosures facing upwards, with a perspex 'window' on top (not sure if its called perspex in the US, but its clear plastic stuff!). The perspex part is open at the ends (so not sealed) and is about 10cm from the cone. My question is wether this will affect the sound quality or not? The persepx is far enough away that the sub isn't going to contact it, but is reasonably close so not sure what effect it will have on sound waves. Ideally I'd actually like the windows a bit closer than that, but not at the expense of SQ. Any advice much appreciated, thanks!