The Deka batteries are marketed for motorcycles. They won't be as strong as the Optimas but they are reported to be decent. I did a fair amount of research into batteries a while back but never ended up buying a lightweight battery.
The Deka ETX14 (the 11lbs one) is rated at 200 CCA but I've read a lot of reports that put them up around 400 CCA. I know they can start AE86s and since the OP talked about relocating the battery to the frunk, he must have a mk1 which is pretty comparable to an AE86. They will probably discharge after a week or a few days of sitting and you wouldn't be able to sit and listen to music with the car off for very long.
I'm curious to find out if the ETX14 can start a mk2 turbo reliably. That's why I had both model numbers handy -- they were the ones I had narrowed my choices down to.
You might also hear about Braille "race" batteries. From what I can tell, these are just rebranded Dekas.