I've got a BED24, and it won't fit on the flat-top with anything EXCEPT the ARMS#40, and even then you can't use the "multi-reticle" feature because the #40 won't let the rear lever move to the two middle positions.
I purchased a Daniel Defense 9.0 forend for my mid-length. I put the front clamp of the BED24 on the forend rail, the rear clamp on the receiver, and it works with any BUIS now.
I wouldn't get an extension because the BED24 is already so tall that the iron sights sit in the bottom of the window. Any higher and you couldn't see your irons at all.
If you don't already own the BED24, I'd look at OKO first. It's shorter, so it fits without a forend, and I personally think the OKO I had was clearer than the Hakko I have now. The Hakko is good, but I think the OKO was better. YMMV
If you DO already own the BED24, then I'd personally suggest taking a look at the YHM forends, as they're good quality at a good price. Daniel Defense is higher quality, but at 3x the price.