I've got a very basic turbo that I got for £20 from eBay for physiotherapy and bike fettling purposes. I was able to bodge it for an ICE Sprint RS (20" rear) with the addition of a couple of bits of MDF from the Useful Bits Of Wood stash and some nuts and bolts. All you need to do is extend the resistance unit mounting inwards until it meets the tyre.
A couple of breeze blocks under the front wheels to keep the trike level, and job's a good'un.
I believe rollers may also work.