On my kit car (a sev7en clone) that does not have a windscreen - therefore no option for a roof, I fitted a microwave detector that is adjustable fitted to the alarm/imobiliser along with an adjustable shock sensor
I have found it difficult to set and position the microwave detector so that it would go off if hands went near the handbrake or foot wells or under dash area but not go off at the crowds that usually gather to have a closer look when its parked up I think my main problem with it being over sensitive is because the interior is just bare polished alloy and shiny stainless - perhaps a carpeted more upholstered car would fare better with microwave sensors?
the downside is of course that I tend to not set the alarm side as people always seem to want to have a good old look around it (and I don't like the idea of scaring responsible and interested people away from looking) as they make a pretty good deterrent and make finding the car much easier in car parks
I have had a clock stolen that was Velcro stuck to the transmission tunnel the bit that made me really laugh is it was the cheapest crappiest digital clock - possibly from a pound shop I still search eBay to see if it turns up as a "very rare genuine Lotus
7 series 1 digital clock with alarm (backlight faulty)"
clock now replaced with a nice backlit VDO analogue to match the other dials ;-)
I think we have very dumb criminals round these parts.....LOL
back on topic , - microwave motion detectors may work better in your car than it did for me - the one I got was very cheap (£18?) and I would suggest getting one that you could adjust the threshold levels on would be a good idea
Regards Rob