the covers are really expensive but they usually have all the air holes necessary & are a good fit if you get the correct cover for your model of hutch. they also let in light as they are clear.
but if you would like a make do cover for now how about a shower curtain over the top i couldn't find a clear one at argos but £2.50 for a cheapy for now for night time?
put a bit of wood or something over the hutch top or bottom to keep the curtain away from the mesh to allow ventilation.
Buy Argos Value Range Shower Curtain - White at Argos.co.uk - Your Online Shop for Shower curtains and poles.
or go to a carpet shop & get a cheap cut off remnant of lino to throw over the top at night & wouldn't go tight to the mesh, which would still allow for ventilation.
try freecycle.co.uk see if anybody has anything appropriate or even an old cover you could have.
i have some clear perspex/acrylic sheet cut off's to go round my outdoor run to keep the wind off. one of them is an old display cabinet screen... doesnt look the tidiest but keeps them drier & more draught free
or maybe try a retail shop (furniture or electrical prob best) & ask if they have any big sheets of heavy duty clear polyethene to dispose of that came wrapped around their wares.
this would let the light in too.
I would probably try an alternative bedding if your hutch is going to be more enclosed, also partly because of the smell given off with the moisture retaining sawdust & the extra risk of eye injuries with dust getting in them.
As Helen suggested, i think fitch is quite popular with members on here.
i have always used sheets of newspaper as a base, covered with shredded paper & then handfuls of hay added. i find this is easiest for me, i have free access to the paper too so the cheapest alternative. also the bedding can be rolled up like a big sausage & taken out all at once which makes it quicker & easier for me to clean the hutch too.
my girls always used to live in a hutch in a shed, i had a duvet cover loosely down over at night to keep out most of the draughts.
@7squeakers. thats a good point hadn't thought of the wood of the hutch being damp... does anybody know of a good product the hutch could be cleaned with occasionally to stop mould spores forming? F10 would that be any good? although my hutch has a felted roof for outdoor use & it would be good for keeping the water off the majority of the hutch anyway.