There will be some hardware needed:

• 20x1mm pipe (aluminum one) - 2m in my configuration, with “horizontal” members about 55cm long and supporting ones about 45cm
• 6mm threaded rod - as many pieces, as “bars”, each about the width of your radiator
• 8x1mm pipe (aluminum preferred, but may be also PVC) - “covers” for threaded rods
• Two closed M6 nuts and two M6 washers per bar
• 14 M6 nuts
• 8 M6x60 screws (Allen key) for radiator clamps
• 8 pieces of PVC 8x1mm pipe (42mm long) - cover for clam screws to protect back side of radiator
• 6 M6x30 screws (Allen key, 25mm should be OK, too)
• 6 small rivets/screws to fasten PipeConnector to 20x1mm pipe (brave people may use glue, but remember, that upper joint works for tension and wet towels may be heavy!)

Printed parts:

• 4 x RadiatorClamp (matched for 40x30 vertical radiator members)
• 6 x PipeConnector
• 2 x PipeEnd
• 2 x SupportClamp ("lower" hole matches PipeConnector, upper hole is used for clamping on 20x1mm pipe and to mount a bar as well)
• 2 x (number of bars - 1) of AlternateBarClamps (they works same as support clamp - tensioning the rod clamps fitting on 20x1mm pipe). You may use BarClamps instead (older design, they need separate clamping screws and nuts - M3x10 + square M3 nut).

Please remember, that radiator becomes hot, especially in the winter. Usual temperature for feed water is somewhere between 75-55C (but some installation may use water up to 95C or even steam!), so temperature resistant material is required - in my case (65-70C on feed) ABS was sufficient (at least all clamps survived the winter ;) ).
4 external perimeters with 0.6 nozzle will give you nice strength margin, infill is not so critical, all parts apart PipeConnectors and PipeEnds do not require supports, as supports will remain inside connectors/pipe end caps, there is no need to remove them ;)
 

Assembly:

1. Cut 20x1mm pipe into 4 pieces - two of them will become “horizontal supports”, the other two will be supporting ones. Of course, each pair has to match its length, there is no “universal” formula as radiators may vary, so a bit of measuring will be needed
2. Mount RadiatorClamps on radiator, secure with screws, remember to use short (40mm) pieces of PVC tube to cover the screw behind the radiator.
3. Mount PipeConnector on one end of horizontal members, secure it with rivet or small screw and attach them to upper RadiatorClamps using M6x25mm screw and nut (Do not over-tighten!)
4. Place two sets of AlternateBarClamps on horizontal members. Holes should be over horizontal members, flat side outside, counter-bored inside.
5. Place two SupportClamps on horizontal members. Clamping side up, flat outside, counter bored/hexagonal nut inside. Then put the rest of AlternateBarClamps (orientation as previous)
6. Prepare supporting members: mount PipeConnectors on each end, place their flat sides in the same plane and secure them with small rivet/screw. Mount them to RadiatorClamps and SupportClamps with M6x25mm screws.
7. Time to take some measurements to determine length of threaded M6 rods and 8x1 pipes:
   1. Temporarily clamp SupportClamps with spare screw&nut, adjust to keep horizontal members as “horizontal” as you want
   2. Set them parallel (you may use uncut piece of threaded rod and nuts to secure them in position).
   3. Threaded rod length: measure to upper outer flat part of SupportClamps, add 6 mm for nut and 1mm for washer on each side (14mm)
   4. 8x1 pipe length: measure to upper inner flat part SupportClamps, add 10mm for counter bores (5mm on each side)
8. Cut rods and the 8x1mm pipes to length.
9. Mount them starting from radiator side, do not tighten the nuts, as it will made installing outer pipes difficult!
10. Place each clamp to its final position and tighten the nuts, so bars won't slide anymore.
11. Install PipeEnds on horisontal members.
12. Done!