I have an Ltuned rear lip, that I bought without the brackets. I too first installed with just 3M tape.
3 months in, it started to become loose, from extreme exposure to the heat (Black Onyx) and the fact that I did not seal the leading edges and water eventually worked the tape loose.
I reattached the spoiler again last summer, again with 3M tape and some extra silicone (black gasket automotive type, between the 3M tape strips). I have no intention of removing it again, so just surface attached it straight to the top of my trunk. You will need to mark the trunk and spoiler, as you only have one shot at this when installing.
I then sealed the entire circumfrence of the spoiler, to prevent water from leaking through and working the tape off again. I left some small gaps underneath for drainage (Ltuned spoilers will have 2 small cut outs on the bottom one at either end), just follow the flow of the design.
It has not moved since, and been through the winter.
Just don't caulk too much on the gap. You just want to create a small seal between surfaces.
Now I don't even think about it, at speed, or sitting in the hot sun.
I only used 3M double side tape, and it's been on my trunk for more than 3 years (including winter) with no problem at all. I guess it's all about prepping the surface before installation, make sure to clean your trunk with some alcool.