As Josh said something to smooth the surface or make it slick can do the trick. Its been put on here before by Scheller and others about using Plexi strips, or others have used trim coil painted to match and not ugly up the home. Bird slide as mentioned can do both species.
In the desert here we have ample night roosting colonies of not only the northern bat species many of you have but some who simply just can't find a better spot than the best looking porches and decorative homes. We've been working on a product we hope to market eventually, but with so many projects who knows when that will come to be. Right now we use our own product and it works well, the homeowners like it and it does solve the problem of both swallows (mostly cliff swallows for us) and any number of bat species we've used it on.
The primary thing we were looking for was to create a slick surface that not only takes care of the 90 degree angle but also a bit onto the ceiling and a bit down the wall. Though we get bats that will sometimes roost down the wall when crowded up higher, most want their backside protected and want to hang at that interface of the 90. With cliff swallows of course, being able to mud down the wall and up onto the ceiling is critical to their nest staying put.
****I would add a word not for those who already have done this, but for anyone who hasn't who sets out to use net with any type of mesh be it truly "bat net" or larger bird type mesh sizes (3/4", 2", etc..). If it is loose at all, you may snag birds or bats and they may die there. While everyone knows for a law to be enforced it has to be witnessed or called in, etc.., I've seen plenty of birds snagged in loose bloused netting that had pulled or ripped, or wasn't tightened recently, and have seen folks who put up simple home depot garden type netting loosely around soffits and snagged and killed dozens of bats.
Conservation wise I don't want those things to happen, let alone the legal aspect, but if you need the legal to think about when reading, understand that a couple of years ago, a contractor for the DOT in California managed to get a couple of swallows hung in his net and passers by saw this and now after he was hit with fines and legal action, several organizations sued the state and they can no longer use bird net in the same way on bridge issues when needing to keep birds and bats off.
Again, not for you veterans of the industry who have used nets longer than I've been in business, but for anyone getting ready to string some up on a first job. Be careful! It can happen to anyone if something fails, but it can happen more readily if you don't assume the worst can happen and have something left flapping or loose.*****
***Also it should go unsaid, but gels (not optical gel as josh referred to, but normal sticky foot, tangle foot, etc..) are getting slammed as well because smaller birds and bats are getting stuck in them and dying where they were made for pigeons and larger birds, even some of those show up all gooped up and folks are noticing, same with glue boards put in environments where people can find birds, bats and other non target things stuck to them, just saw a pick of 10+ chickadees on one wasp trap type that was from a colleague in WA state. People are pressuring stores and manufacturers to change the products or put warnings, etc... or discontinue them altogether. Worth knowing and keeping in mind, I've had folks get their company tagged even in my small state over these and I don't think any company wants to be on the paper or news for some of this.*****