i'd recommend jumpers horse line ones. they aren't too expensive and really last from my experience. lovely long necks and plenty of shoulder room- perfect for both my cob and tb!!
Ive had Derby house rugs in the past and really rate them. Had a heavy weight that looked very similar to the Mark Todd hw and it never leaked and fitted really well, kept her super warm through the coldest weather. Not so sure on them now they have changed the design slightly but i should think they are still good.
I now use JHL rugs and love them, fit really nicely and i really like the full tail flap on them.
i would look at good quality 2nd hand rugs via ebay rather than buying a cheaper new one especially if she is tricky to fit as some of the cheaper rugs are not so generous round the shoulders/neck.
I have found that the fit has been the issue on the cheaper rugs for me rather than the waterproofness.