The common fix is to inclip the black cylinder things that hang down either side of the boot lid, and put a penny in them before clipping them back on. This then makes the gives the effect of making the springs stronger...
BUT... your method may be better, as the black cylinders are quite weak and i've broke the housing on mine when adding the 1p, and had to replace them, and i know IanS16 mentioned that it had damaged his hatch or something?