you can force a non-ideal alignment into IB with an aperiodic (AP) membrane and get good results. I've tried various woofers in IB/AP and all did well if your goal is accurate music. I wouldn't try SPL competition with the alignment.
there are a variety of ways to implement AP. this is a good resource for the various methods:
AP Enclosures-The Aperiodic Cookbook | Tutorials | Team Audionutz
GlassWolf's Pages
i did mostly rear loaded membranes (or chamber-coupled)
but your best bet for least work is front loaded. just sandwich the fiberglass (or preferably ultratouch insulation) between two speaker grills, secure to the rear deck (needs an air-tight seal against the woofer basket) and experiment with density for tuning.