I didnt buy any commercially available thing, but the same idea: I used some "die springs" with washers on both ends. About $1?

Later,, I had go remove a spring from one side because it had too much tension and would be hard to close. A single spring was still enough to pop the trunk open by itself.

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Mike - Greater Seattle, WA, USA
2002 E53 X5 4.4i Sport Pkg Black (daily & tow vehicle)
2003 E53 X5 4.6is Estoril Blue (to replace 4.4i soon)
2002 E53 X5 4.4i Sport Pkg Silver [sold]
2001 E46 325Ci Hardtop 5MT Topaz Blue [sold]
+ Toyota track/autox/rally cars: MR2 (x2), Celica AllTrac Turbo (x3)