Have a search over old threads, I can't remember the name of he half cage Greg put in his 2litre...
Chris even tried to organise a GB or discount on them donkeys years ago...
Why you would want a half cage I do not know, been supported from a singe side, the rear of the car means if hit forwards it will just fold in.
In a crash they can do more damage than good when not properly designed or installed and more so when you don't use a helmet, which you don't in the road???
If your adamant, try Roll Centre, they may be able to make you a custom made one, short of that there are dozens of companies who can fab a a cage from scratch but you really need a quite bare car...