Here's  a round-tube rear frame clip ready to install.  It's 3D printed in FUD material (see below).

Dimensions are:
Frame width = .700" (about 45/64", 17.8mm)
Frame tube diameter = .080"
Cross member diameter = .063"
Overall length = 2.55"

Two other size frame clips are available in .800" and .900"

To complete this rear assemly, also available is:

• 4-link set designed specifically for this rear frame clip
• Coil over shocks
• Dana narrowed rear end and brake set
• Fabricated rear end
• Narrowed 9" rear end and brake set

       Photo of actual Round Tube 4-Link Rear Frame

This is made with Frosted Ultra Detail material.

The 3D printing process leaves small 'jaggies' on trailing edges of the rails which can be sanded like normal and then primered.

 By Futurattraction, 1/25 scale.