E28driver,
That's a neatly done job there. The idea is to sandwhich a pod like kickpanel (MDF) over the current stock - adding depth which will allow for a larger driver like a 6 inch mid to fit. Check out the finished look on page 3 of your link. The hood release lever can be just moved a couple inches sideways - not sure how it was done but I've seen a couple finished examples in E30s before.
Some slim magnet drivers can be found in the range of comps offered by Focal, DLS, JL, Eton, Boston etc. Ideally, anything with less than 3 inch mounting depth shud work just fine.