I use the Blackburn Outpost Fat Rack. You can install it with P-clamps in the absence of dedicated frame mounts.

 

I like the Topeak fat rack, but I'm pretty sure they stopped making those at some point in the past.

 

I was able to get one recently for my E06 (wanted OMM but was always out of stock). It is a nice rack and will work with the RAP axle with some longer bolts. I got the kit from RAP that included the axle, seatstay braces, etc and did a hybrid.

I used the OMM extenders, trimmed off with a couple of holes drilled for 9/16" U-bolts as I liked that setup better than the Topeak braces. The OMM pucks probably would have worked fine but I wanted something a bit more secure so I made my own.

It has held up perfectly so far, even over some really rough terrain with a rifle cantilevered off of it.

 

Old Man racks work for some bikes without attachment points.

 

Might add a Problem Solver Seatpost Clamp mount to go along with the Robert Axle.

(image stolen from Problem Solver website.)

Axiom has a Streamliner Fatliner, looks to be what you want.

(image stolen from Axiom's web site)

I'm a fan of the Axiom racks. I've been using one on my gravel bike, on and off for a while now. Customer service is great too.

 

Might add a Problem Solver Seatpost Clamp mount to go along with the Robert Axle.

Salsa also has the rack lock

Salsa Cycles | Adventure by Bike

www.salsacycles.com

 

The Robert Axle Project extend axle is one way to go but kinda pricey.

Old Man racks work for some bikes without attachment points.

RAP and OMM are the same company now -- right?

Regardless, it's a top notch solution.

 

I was in the same boat (no attaching points), I went with the OMM, I really like the way it mounts and it is certainly beefy (70 lb rating), but it is on the pricier end. It was the only system I could find that I was positive I could mount up without any mods. The Blackburn may have worked with the RAP axle, but I couldn't be certain without actually testing it.

 

I just remembered......a couple months ago, OMM updated their rack, there is now an over-seas made rack and an American made version, you can save some bucks on the over seas made version.