I was watching a John Mills (Doubleboost) video on youtube and I saw him using a little machinist type jack on his mill. I am not sure you can call it a jack or maybe an adjustable parallel. I liked the idea so I made one this weekend and it seems to work ok. I had no dimensions so I looked what I had in my little home shop and used what I could find. It is very small as I made it from a 25mm x 25mm piece of square stock. It is 58mm long and it extends 10mm. Once the lock down is over and the shops are open I will be able to buy some bigger stock and make one from 40mm square stock.