Was actually thinking exactly along the lines of what you said - get something in between the fan and rad somehow.
My main worry is that adhesives tend to weaken when temperature cycled. In my experience at least.
I'll do my best to give some input, but it's hard for me to contribute without being in front of it, or at least some more pictures.
I had in mind something like the following:
Parts
- Plate aluminium, it's easy to work with hand tools, and bend without heat. Will take a key and paint too. Thicker is sturdier, but harder to work.
- Radiator Gasket, this will take up the 1 to 2mm apx space between rad and fan created by the brackets, it will also help to decouple from the case. Might need a trim.
- Paint & primer, to match that lovely colour scheme.
- Optional; Neoprene sheet, 1mm. Or more. Just would help decouple the bracket from the case, mounted to the bottom of the bracket. It's often adhesive backed.
Concept
An s-shaped bracket mounted to the base of the case, other end fitting between the top fans and rad, suspending the assembly off the case floor. Neoprene rubber can decouple the feet from the base, a gasket and spacers can ensure an even seal between fans and rad.
Make up a couple of brackets, one for each end of the rad where you've drilled some holes. Two 90 degree bends to make an S shape, the rad mount trimmed so it doesn't block any airflow.
Process
1. You could start with a really basic rectangle shape, then make the first bed about 10mm up for connection to the base. Then go through the bottom to mark the mounting holes.
2. From there measure up the bracket the height of the rad and bottom fans, plus the additional clearance required. If adding the gasket and neoprene to the base of the bracket, you need to account for these thicknesses too.
3. Make the second bend at this point, for the part that fits between the rad and too fan.
4. You'd need some trimming with a Dremel / shears / files to match the profile of the radiator mounts, and not impede airflow by blocking rad space.
5. Either get some spacers the same thickness or the plate, or make up your own from the scrap. Fit the gasket to the rad and note any parts that might need trimming out.
6. Once it's tickety boo you can take it apart and paint it (key your surfaces, and clean them) and you've got a solid, unobtrusive mount.
Finishing Thoughts
There's a million ways you could go about this, for instance single, small brackets for each corner, and the middle. Side brackets that run the full length.
You could even just get some m4 stand-offs and just bought to the base of the case!
You're spoilt for choice, and every one leads to another opportunity to have a good go at that case of yours
Or course this all makes complete sense in my head, but you may think I've lost the plot.
Whatever you do, my suggestion is do it for the fun of it. Personally I used to always worry about getting things wrong, or 'ruining" a case that's just for me, and is easily replaceable.