I made a pair last year which worked well, so I hope to make a few improvements this year.
Ir bandito’s photo shows good positioning of the screws, if anything I found they can be spaced even further apart (around the circumference) without any loss of bite, but it needs to fit the tread pattern you start with.
I would advise using the shortest screws possible, they only need to be just proud of the rubber. Despite a few well gritted sections on my commute where the tyres rattle along on the tarmac, the screw tips did not wear hardly with a couple of weeks use.
Preventing punctures was the biggest problem. I used duct tape and an old inner tube split open to line the inside of the tyre, but still got punctures after 3 or 4 days use. I used countersunk screws rather than roundheads which may have been better. A half decent tyre to begin with helps, one of mine was too worn out allowing the screws to flex too much in the carcass.
Riding is hard work, but there is a sense of achievement riding past BMWs and Mercs with ther rear ends stick in the gutter, and being one of the few people to be able to get to work on time.