the best thing that you can do with your $9 would to buy a small square of dynamat/secskin and dampen/seal off the rim of the mounting hole. the more you can seal off the front of the speaker from the back of the speaker the better. preferably you want to seal up the whole door. many car speaks are made for infinite baffle installs. when you constrict its air space when it shouldn't be, it can cause distortion.
Got a pair for my front speakers (Infinity Refs) and something else must be wrong because my speakers sound like **** in the midbass frequency range.
I have a set of Inf. refs without the baffles and they are rocking a healthy amount of mid-bass\range running off of a alpine 50x4 bridged, gains just barely above minimum. no dampening. are you running an amp?
Toreador Red 05' FX4
4.0 4x4 A/Tran Ext.Cab Daily Driver. Nomads, BFG M/T's
Alpine 9857, JBL MS-8 DSP, US Acoustics 2x80w & 2x100w, Vifa 1" XT25 Dual Ring Radiator Tweeters 3k+ 24db slope, Dayton Audio 7" RS180-4 Mid-woofers 100Hz-3k 24db slope, 500w rms Kenwood Class-D Mono - JBL 10" GTO1014 Sealed. 100Hz- 12db slope