I ordered the Product “heat booster” a longer while ago.
https://www.ebay.de/itm/165847307968?mkcid=16&mkevt=1&mkrid=707-127634-2357-0&ssspo=pv7wiebrtl6&sssrc=2047675&ssuid=K0KR8BFNRKO&widget_ver=artemis&media=COPY

They are pretty expensive and VERY LOUD, loud enough to send them back.
So I thought “I can do this better”, so I started to design the same, but not with a proprietiary power-plug but instead with a simple USB-C supply. USB-C is also used to interconnect two of them.

You need a couple of items to produce it, but at the end, I was able to get the total cost for a set of two down to about 65€, compared to the readily available ones. This is still about only ⅔ of the price for the readily available ones.

They are about half as loud and will run with any USB-C power-supply (also 5V, since we use a step-up converter).
The noise is even more important than the cheaper price. The fans I've chosen have a magnetic bearing and should last for very long.

Printing is pretty straight forward, for the housing you need supports, same for the magnet-holders.
I used PETG to print the materials, since it is temperature resistant enough to keep up even with higher radiator-temperatures.

Parts you need (for a set of two, like the offer from eBay):

1. Sunon Fans, 60x60x25mm HA60251V4-A99 DC 12V 2500 U/min 11dBA
   6 pieces https://www.ebay.de/itm/364074530887
   
2. Type C Board with step-up converter to 12V:
   1 piece https://www.ebay.de/itm/275268770666?var=575537855882
   
3. Screw with TORX-Head DIN 7982, 4,8x13mm (Also 24 will do, since only using 2 screws per fan per side is fine)
   48 pieces: https://www.ebay.de/itm/251926953246?var=550803832628
   
4. USB-C Connector
   2 pieces: https://www.ebay.de/itm/284877532294
   
5. Temperature switch KSD9700 @35°C, normally open:
   1 piece: https://www.ebay.de/itm/112436515387?chn=ps&var=412827500227
   
6. Magnet for the fan-housing, 12x5mm:
   8 pieces: https://www.ebay.de/itm/232756537858?var=532367452360
   
7. Magnet for the temperature sensor, 8x3mm
   2 pieces: https://www.ebay.de/itm/232756537858?var=532270249173
   
8. Cable 0,25mm² or 0,35mm² twin conductor
   about 0.5m: https://www.ebay.de/itm/202126836931
   
9. USB A to C cable, 1m
   1 piece: https://www.amazon.de/gp/product/B08ZY49PLX/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&psc=1
   
10. USB-C cable for interconnecting the two units, 0,3m:
    1 piece: https://www.ebay.de/itm/394527800138
    
11. Power-supply: The cheapest 5V USB A-supply will do. Even with only 2A and no PD or whatever fancy stuff will work perfectly

Wiring: Pretty easy: Solder the positive and negative to the step-up converter, only the positive is in-between the positive wire. The end of both wires go into the breakout-board.

For the second unit, only all positive and negative go into the breakout-board and to all fans. (no step-up converter and temperature sensor needed)

The magnets will be glued into the temperature-sensor, same as the sensor, which shall protrude slightly to make good contact:


The other magnets will be glued into the magnet-brakets (print 8 pcs) and these will slide into the housing. There is no retaining clip, so they have to be put in manually and you need to hold them in place when sliding the unit into the heater / radiator.

The boards (breakout USB-C and the USB-C step-up converter are just mounted to the holes with hot-glue or superglue (CA-glue). Make sure to glue the properly after a function test, it has to keep up with the insertion-force of the USB-C plugs.

Have fun and save some energy with a lower water-temperature while maintaining your comfort :-)