We had a livery that used a Boa boot and it was great. Horse had poor feet and it was used for turnout when we couldn't get a shoe on him. It has a lace system that you tighten with a little disc at the front. It is almost like them wearing a trainer Think they are quite expensive but have noticed them on equestrian clearance website. You need to make sure you get the right size too.
I seem to remember someone saying that they had made a very effective boot from tyre inner tube and taped onto the hoof. Cut larger than the hoof and then cut down in spaces to hoof size and fold the cut bits up over the hoof itself and secure with gaffer tape. Practically free I think. Worth a try.
I've tried many off the shelf boots in the past and I have never found one that doesnt rub the heel. I tend to rely on making my own with lots of gamgee, and the plastic wrapping off haylage and lots of silver duck tape.
When my mare had a foot abcess and had to stay on the yard for months to keep the mud out I had great success with the blue stiff plastic that is used for damp proof membrane. You need to cut out a circle and then do loads of cuts in it to fold up the hoof before you tape it all up with duck tape. Obviously need gamgee under to stop rubbing but I found that I could take if off an re-use it about 4 times as it didnt wear through on the concrete and foot stayed lovely and dry.