Perhaps wooden shutters installed outside would work. They'd look amazing and fit in with the look of a barn conversion too. Then, you could still consider the louvre drapes for the inside, but keep them open, only closing when you want privacy. So in this way, they are really being used for 'dressing' purposes only. The reason I'd put wooden shutters in, is that you wanted to dress your window, and since this is a barn conversion, the window rests on the floor and is not conventional, it calls for unconventional dressing. So, shutters will add interest and depending on how high they are from the bottom, outside, they could be used to close up for privacy, but if too high up, then used only for effect, and the louvre material drapes inside to match your décor. You could use short curtains, but as I said before, you'd have to tie them back so that your window can open, which would work find as well. perhaps draw it out, using the various options that you've been given by everyone, and see what appeals to you? You could make this window a feature, which is what I'd do, instead of 'just a mere small window' and really make it a 'wow' feature in your room! don't be afraid to be brave with your décor on this one. have fun with it!

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You can get blinds that clip into the upvc window seals that could be effective here in the awkward space. Lots of places sell them. Not the most attractive but they d the job :) https://www.blindsdirect.co.uk/perfect-fit-blinds?gclid=Cj0KCQjwi7yCBhDJARIsAMWFScOOvl0wjY6YJjqrSY36htfwui-z_qtOKBDtqV__gAWxEXAQj6rNw0IaAhivEALw_wcB 

I buy the mechanisms and make the blinds myself . I never make recessed ones as they pose too many restrictions and I personally don’t like them recessed if they are the primary window covering . I’ve never bought ready made ones so cannot comment on quality . However I know they can cost a fortune if you have them made to measure from blind companies . They are not difficult to make and use a fraction of the material you would use for curtains , however they are time consuming to make . Look for a local soft furnisher if it’s not something you are able to tackle. They would , hopefully , be more realistically priced . If you do go with blinds 2 go , then as macbroom says read their measuring guide very ,very carefully . I didn’t when ordering a roller blind from them and discovered that the width of their blinds is measured from end to end of rail not width of material , something they don’t make clear .