As pre Rob Holland they are a bit of a pain to fit but once on they are great. We've had two over the years, one for my son and now one for grandson, the latest one does have a QRC at the top by the seat post so if you're not needing it you can quickly remove the majority of the trailgator and it just leaves a little attachment on the post.
My Son and I used to use ours for XC rides all over the place, he got so good on it, he used to ride along standing up on the saddle and handlebars, but it meant when he got worn out we could carry on much further than if he were on his own.
Is it a Raliegh MX16 you're talking about?
If it is I just googled an image and it looks like it should be no problem, they have several holes to fit the locking piece and I've fitted them to three bikes so far with no complaints. We paid £44 delivered for ours by googling around.
We've just spent two weeks riding all over the place here in France with my 4 year old grandson on the back, in two weeks he's now riding a little without stabilsers, and is far far better balanced when on the trailgator.