I've made a small batch of manual boost controllers, pictured below. I've been using a slightly less refined version for the past few months and it's worked flawlessly. This is the Mk2 as it were, all made from high quality 1/8" BSP fittings.
Price is £16 + £1.50 P&P, buying new, you won't get anything of comparable quality.
PM me for further details.