i didnt use adapters i just mounted them with 3 bolts pushed it all the way to one end and screwed it in and i wouldnt suggest mdf in the door like that cuz water moisture does get in there and they'll rot away pretty quickly
Hopefully you sealed off the large hole that was left. sealing off the immediate area around the speaker with a solid non resonate baffle is extremely important.
You will loose a large range of frequency response in mid-bass range (500hz down) if you don't
there will be a sonic hole in your system because the backwave of the speaker will cancel out the front wave in that range.
MDF is fine as long as you seal it with something paint, bedliner, polyurethane, etc...
HDPE is the stuff that plastic cutting boards are made of. you can find them almost anywhere.
adapters are only $10,
the best solution is which ever gives you the heaviest, stiffest, non-resonate most airtight results. (ive seen people use granite rock before, whatever works)
since were on the subject, Sealing off the rest of the doors openings with sheet metal and sounddampener will improve sound further. a closed cell foam mat behind the speaker will absorb the backwave, which will clean up the sound even more. Speakers in a tin can, sound like speakers in a tin can.
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