How about Robin Hood?! You can wear brown jods and a green tee shirt - and making a bow should be easy - hunt around for a large bendy swig and put some baler twine on it! Oh, and for a hat you could either use green felt (cheap from cloth stall on market or habadashery shop) - I don't think they are hard to make - even with my clumsy stitching I'd have a go! Just make sure its big enough to sit over your proper riding hat. And for your quiver of arrows you'll need something - preferably brown coloured to match your bow. I'll keep thinking.........
And of course your horse would just need some medieval looking saddle cloth / reins.
Whoops! That was meant to say twig not swig.....hic, it is Friday afterall.......!!!