Light weight enclosure material?

those are hardcore heavy duty subs and as per JL website you will need significant bracing to handle the pressure these subs create.

even 3/4 inch MDF when braced will be a VERY heavy box- not too heavy to lift but too much to drive anround with if your suspension is tuned to race or rally. you will kill a good suspension with 100lbs of mdf on the back axle

remember those subs are about 15 lbs each bare- and you weill need a pretty heavy duty box to hold em otherwise without a solid box for backpressure, you will kill the subs. they need a good box, which you dont want to put in for obvious reasons. Honestly, sell the JLs youll get good $$ for em and put in 6inchers- the box will be tiny and light and still shake your car.

Just my 2cents- Good luck!