I've been working on a prototype seat bag for several months now - this is my third version. Brandon will get to test it out in a couple weeks on an overnight bike packing trip the shop is doing. Unfortunately I get to drive the SAG vehicle because I recently broke my ankle and I off the bike for at least 5 weeks (hopefully that will give me enough time to train for the Fitz-Barn once I get back in the saddle).
I've also been doing these gas tanks for a bit now as well, but I haven't shared any photos. So here they are - pretty simple with fully adjustable velcro so that you can fit the velcro around any frame bag attachments.