I thought I'd compile a brief list of the bits I used to build a magnet speed controller.
1. https://www.ebay.co.uk/itm/5V-16V-10A-PWM-DC-Motor-Speed-Controller-Regulator-LED-Dimmer-Speed-Control-UK-/392955429219?_trksid=p2349624.m46890.l49292 I used a similar one to this, shop around and you might be able to save a quid or a few pennies this was just one that is pretty much the same as what I bought.
Other bits include
Wire, terminals, heat shrink and an inline fuse holder.
The build is fairly self explanatory. Just drill hole in box, make sure its in the correct position otherwise the lid won't fit. Insert speed controller, do nut up and replace dial. Drill hole in the other end of the box, make sure it's correct for the grommet you have and wire bundle. Thread wires, solder fuse holder and terminals and Jobs a goodn. I prefer the blade fuses to the glass ones as they're less fragile and easier to obtain. I did have some heat shrink, solder, terminals left over from wiring my camper so it did only cost me £10. You could do it cheaper though if you just glued the wires in and ommited the grommet, used an old kitchen tupperware for the box etc.
Hope this helps someone save a few quid to buying one. If anyone knows where you can buy the electronic board for a random delay flapper timer, please share as that the next project.