Pretty much every option there is involves removing the FSB and swapping the circular handguard end cap and replacing it with a triangular piece.
Making triangular handguards that would fit a round end cap wouldn't be impossible, and I am curious sometimes why it hasn't been done yet - the triangular carbine handguards are from what I've seen - relatively slow sellers. However, I suspect that part of the slow sales is because the number of people who want to go through the trouble of removing the FSB just to mount another set of drop in plastic handguards is pretty small.
If it was merely a drop in replacement, they'd probably be more popular - observe the MagPul MOE handguards. For the life of me, I can see little difference ergonomically between them and triangular handguards - aesthetically they look cool to some people - and the polymer might be a little more advanced - but ergonomically, they're very similar. Now - given the long availability of triangular handguards, and the sudden explosive popularity of the MOEs, I'm curious as to how much of it is merely MagPul love for the sake of MagPul love (which is not to say I dislike MagPul products, I have many of their products that I like, and the good ones are damn good, but they do have something of a "following"), but I digress.
With that being said, *if* OP, you're looking for a set of triangular handguards for the ergonomics and not for "the look," I might suggest the MOEs.
~Augee