Most of the folks that I know who have made minimash tuns have simply for a little SS braid. Just cutting one down and folding over the end of it. I think that is the way James @ basic brewing did his. Even chubbykid brewing did it for his 5L minikeg tun
I just us a folding steamer in the bottom along with a grain bag. Just break off or unscrew the center post.
One of these, it helps to lift the grain bag above the spigot to keep the drainig from getting stuck.