The SLK from new had an option of a roof rack carrying system although doubt this was ordered on many cars and also not sure it could be retrofitted.
My previous car a BMW 330 convertible I had a removable towbar fitted (completly invisable) and could attcah a rack to this.
I also had an mx5 years ago and did look at the (at the time) Paddy hopkirk boot racks which you could fit to boot, remove front wheels of bikes and secure. Will not fit however if you have a spoiler.
1. The type that clamp round the ball (Thule call them hang ons) - you generally need reg plates behind your bike too with these to keep the stroppier cops happy.
2. The type that bolt between the part attached to the car - and the tow ball its self - this type means you can still tow when using them - but means you have permanently visible towing kit.
3 The ones like platforms/cycle mount bootracks that clamp into the tow ball. These tilt so you can open the back of the car and offer up to about 70kg carrying capacity sitting behind the car. The also have plates and lights fitted too.