I'm looking at this one...
12V 30A 360W DC Motor Speed Controller with Enclosure | eBay
...more expensive but it ticks all the right boxes for me. Right current rating, right frequency (ultrasonic), comes with a case and can be controlled using a simple 0-5v signal, which can be fed directly from the existing speed selector switch (with judicious addition of a few voltage-dropping diodes) without running any new wiring or requiring the addition of an extra control knob on the dash. Also, the listing shows that the seller understands what he's selling.
But your blower motor is almost exactly half the power of mine so maybe you don't need such a high-rated controller. The lower-powered ones are MUCH easier to get hold of. Also, if you wire it up clever then for the highest speed the current doesn't have to go through the controller at all but can bypass it. That way, the maximum current you put through the controller is going to be for your second-highest blower speed, whatever you decide that will be.
It is entirely permissible to rig it so that the three controlled speeds are slower than the existing three controlled speeds and then there's a jump up to the maximum, uncontrolled speed. That way, the maximum current you ever put through the controller need only be about 5A. (Or for me, 10A.) I myself very rarely use speed 3/4. I sometimes use speed 1/4 but mostly use speed 2/4 for everyday use, ...and then very occasionally, when I need all the power I can get for a brief period, I'll use speed 4/4. I'd probably be happy with just two speeds but that would make the speed selector a bit weird. Just thinking aloud here but in fact I could use a much lower-rated controller (say 15A, which is readily available in the UK) and have in effect, speed 1, 1.5, 2.5 and 4.