I recently purchased a Mapex drum riser for an 18 inch bass drum.
I use a Premier Heritage kit and although I like the sound of the kit, I wanted to raise the drum so that the beater was more central to the head.
The Mapex Riser has done this very easily. The drum is at a height of a 20 inch bass and the riser is easy to fit to both the drum and pedal. It is lightweight and can easily stay attached to the pedal for ease of transport.
I would certainly have bought one sooner had I realised they existed.