It wouldnt be that hard to make really. Youd have to get a recirculating tubular o2 housing and cut it on the recirc tube but not in a position to cause turbulance. Get a flapper and assembly off a small 2stroke carb that will fit on the whole you cut. Make something to hold the flapper and weld it on, as well with the rest of the dump tube pipe. You could if you wanted hook up a small wastegate off a conquest and a shitty mbc or garden valve and set it so that it will only open when under high boost.....just an idea, im not sure if this will work as it only took me a minute to come up with lol.