For the last few weeks, I've been doing lots of testing of NetBSD's build.sh cross-build system on lots of different platforms. Linux is readily available on AWS, as is FreeBSD. You will find NetBSD and OpenBSD in some AWS locations. It's more difficult to get the BSDs onto AWS because the standard upload tools detect the filesystems and if they are not on the list, the image is not allowed. The BSD FFS and variants are not on the list.
Fortunately there are other tools and other ways to build images. It's a little protracted. You need to first build a VM, convert it to VMDK, upload it to S3, create a snapshot and then convert it to an AMI.
This set of scripts (by Antoine Jacoutot) does the hard work for you and works for OpenBSD 6.5 up to 6.8. Last night, I created my own fork here which also works for 6.9. Essentially the difference is that the OpenBSD install kernel is compressed and the script now decompresses it and recompresses it where needed.
So as a result on AWS eu-west-2 (London), there are are AMIs for OpenBSD 6.5 through to 6.9. Just search for OpenBSD when you want to launch an image.
There are NetBSD images out there but I'm hoping to get around to producing some too.