More questions:
Do they protrude down into the rear bins?
What sort of volume are they?
Mine goes down & takes up as much of the rear bin as I could get it in to (still leaving room for the belt) and is about 16 litres. It was a pig to make because of all the differet angles, but I used 9mm mdf and fiberglassed te inside just like yours.
At the minute they don't and am thinking it'd be a wise move. tho sometimes you can hear the side of the car vibrating

They can be on the scale of loud to loud as .... they are on the getting close to loud as.... end. very very good little things even with their small sized magnets.
I know what you mean about the angles, it was a case of cut abit and measure up. Took me all of a day to make em both. But I still got the belt in there. The rear belts are bolted in on the rear bulkhead so I dont have to worrk about those.
And in the boot, 2 more of them in separate boxes as it means an easier job to get to the battery and spare wheel. thats the only issue with boot jobs. If you can't get to your spare wheel very easily, then try something else.