Alternatively you can by some tiny 4" Woofers, to move your bass up front. I know i had some fitted in my t4 van which has similar size hole, you'll need an encolosure of sometipe, i resorted to taking a yougurt pot, big onken one, cutting it down and covering in some fibergass resin and wading. little 4" subs are rare but option worth considing. bit of wasted space when you can mount your tweeters anywhere. I mounted my tweets in pillar, it think the pillar plastic i butchered was audi 80, but can't remember now. When scrapyards were the place to rummage and find something to do the job rather than being shown to one particular car!
