The difficult places to cover on a horse are the belly, legs, udder and sheath. We've found two companies, so far, which aim to solve these problems. Solva and Snuggy Hoods. The alternative to covers is to spray with insect repellent, preferably a non toxic Saltidin/Picaridin based one. Avoid Deet, if you can, as it is toxic; and we're not so keen on Pyrethrin-based insecticide either which is reputed to cause an allergic response in quite a lot of horses. Professor White says 20%-30% of the problem horses seen by her were allergic.
Solva Icelandics udder cover is customised for full size horses, ponies and cobs in dark brown and olive Solva offer to sew it onto a new rug. You attach it to an existing rug by means of velcro and clips The latest design has adjustable straps. Solva don't provide sheath covers at present as they've not found a way of overcoming the inherent problem of mesh size small enough to exclude the midges and large enough to drain pee through the cover. You'd best see how Snuggy deals with this problem, if at all.