Life Lesson: Don't buy the accessories before you buy the bike. My... assumption is your gain! 

My planned bike purchase has changed and I no longer need this rack. It has been mounted on another bike just for safekeeping but was never ridden or used so it is in LIKE NEW condition. This uses Topeak's MTX system for securing your MTX Bags. It is super secure. I will also have the optional side panels under a different listing if you need those for a panier type bag.

From Topeak: 

Description:

For M, L size frames

The newly designed, extended, and widened version of the MTX BEAMRACK is better suited for 29” MTB and gravel bikes. It features an MTX QuickTrack® panel, compatible with MTX 1.0 and 2.0 system bags and baskets. Flip the panel to adapt it for bags and baskets from other brands.

MaterialAluminum
ClampFits ø25.4 - ø31.8 mm seatpost (shims Incl.)
Max load9 kg / 20 lbs
CompatibilityTOPEAK MTX 1.0 / 2.0 Bags and Baskets
Size54.1 x 14 x 11.9 cm
Weight673 g