One of the reasons for not having a back pack blower in my 1st 5yrs in this business was not knowing how to transport it within the rig without rolling around and tipping over along with ensuring it wasn't going to get stolen.
Was operating with single cab Hilux and caged metal sheet sided trailer with front enclosed mower box and full height roadside tool box.

Then came the Dual Cab utilising same trailer and the solution for me became, laying it across the rear dual cab seat, but it needed securing to stop the unit flipping over or sliding of the seat. The Sthil BR 600 sat well, but the newer BR700 being slightly higher and deeper (ie closer to front seat than Br600 when positioned on back seat) needed a harness.
This is the dead easy simple solution and is secure.

Click open eye schackle onto the passenger side mini head rest chrome post mount, push head rest down flush with top of back rest. The Velcro straps are quite long and loop through top plastic handle on Blower unit and Velcro strap back.
It doesn't pull on the head rest stem, just provides the unit with stability, forward and side ways remembering the B/pack is only some 10Kg.

Works a treat and the straps at Bunnings and cheap whilst coming in varied sizes and Velcro lengths.
Plus I've black canvas custom fit seat covers front/back and lay across back seat a crappy old beach towel the seat for quick removal when not working and need the back seat which is not often. due to daughters being adults