When I had my '75 K5 ,that had a roll bar,I bought a "tube" speaker enclosure from a flea market vendor,who made them--it consisted of a 2 foot or so long length of 8" sonotube,with a circular center "divider" made of 3/4" plywood,and two end "mounting rings" made of the same plywood,that screwed to the sonotube ,so the speakers could be screwed to them..also had a 2" duct port in the center of each speaker chamber to release the bass pressure..it also had a horn tweeter in each speaker chamber with a capacitor for a crossover..
The thing sounded awesome with a amp/equializer,lots of bass,and treble from the tweeters...I used giant sized zip ties to mount it on the roll bar,but I later decided to use perforated metal strapping instead...
I liked those speaker enclosures so much I ended up buying 4 of them,I still have two of them,using them in my room now..they were all slightly different lengths,I found the shorter of the 4 to have the best bass and overall sound quality..