I'm looking for an AR15 mid length (existing handguard measures 8.6" overall) M-Lok or quad rail type handguard which works with a picatinny gas block (clamp on), triangle type handguard cap, and the spring loaded ring retainer.

I was going to go with MagPul, but according to their customer service, the ones they offer in the length I need aren't compatible with my triangle cap.

Who makes a good handguard compatible with the above components?

Thank you!

MWI has Mlok and quad rails for that.
https://www.midwestindustriesi...uct-p/mi-18m-blk.htm

I am probably going to get laughed at but on one of my inexpensive ar rifles, I have a Leapers/UTG(Under The Gun) Pro-Slim Metal rail/hand guard. I have to say it is actually pretty nice and is made for the triangular fit. It is a 2 piece drop in part for the mid-length guns.

Something like this?

https://www.midwayusa.com/product/102169157

If your willing to spend, the new Geiselle handguards are top notch.

Thank you for the suggestions,everyone.

I ended up going with one from Bravo Company.

Fitment is very tight, it doesn't move around and feels pretty sturdy. I've added a MagPul AFG2 angled grip and quick disconnect for thean MS4 one to two point convertible sling.