you said
I'd like to use for HFT competitions but need a single shot magazine.
i shoot HFT and use a magazine with me FX Cyclone. There are no rules against using a magazine. I find it more convenient to use a magazine. I can load it between lanes and then examine the skirts to see if any are bent. if so, i push it out and load another pellet. when it comes time for shooting, i don't waste time fumbling for a pellet. since you only shoot 6 shots max per lane (2 per target), the 8 pellet magazine works just fine.
I don't have a single shot tray, but if i did, it would be to try the pellets, such as Predators, that don't fit in the magazine. but frankly, those type pellets are gimmicks and i wouldn't shoot them anyways.