is the omlet going to get more draught
It is certainly more likely to if the varroah board stayed in place and the wind direction was parallel to the hive as the board does not prevent the draught being directed towards the floor area, but may actively induce a draught. How much the OMF dissipates any draughts is another matter.
I, for one, would not be leaving that particular board in place (however long it might stay there!). For a start, it is no longer a varroah board as most debris will be blown away and secondly for the reason above. I would recommend a home-made board that fitted properly and securely and with offset ventilation, with regard to the cluster position. I would also be scrutinising the top ventilation and possibly be taking steps towards reducing that.
Those plastic 'book/leaflet/booklet spines' are a jolly good idea for making dividers/dummy boards for any hive as there is a small amount of adjustment available on the dimensions to achieve a snug (or an accurate) fit, they are chicken feed money from any stationers/art shops, easily sized and disposable (recyclable). Full marks to omlette for that particular part of the design (or was it simply a fudge to avoid some of the issues?). Hope it is not copy-righted!
Regards, RAB