After looking at the replacement part prices for the pot-metal crap that bomar uses in their risers I decided to make my own assembling parts available from McMaster-Carr. I think the small tubes are 1/8 and the large tube is 1/4 inch stainless pipe. The end is a pipe cap and the other piece that holds the hatch in various positions is a "hand retractable spring plunger" that locks into holes drilled in the small tube to lock the hatch into any of several opening positions.