I guess it depends on how much stick out you can allow. I would use spindle liners in any case. I made mine out of aluminum tubing and then made plastic bushings about 4 inches long to fit inside the tube, then just filled the tube with them, I used a light press fit on the bushings. I chose 4 inches because that was easy to drill through, and drilled them 1/32'' over the material size. I have a liner for each size that I run. I have o-rings on each end of the tube to center it up in the spindle bore, and relieved the area between the o-rings for easy insertion.
The spindle liner sticks out of the spindle about 8 inches, and I allow about 8 inches of material stick out beyond that, for a total material length of 48 inches. The smallest material I run is 5/8'' and the largest is 1.5'' My spindle bore is 1 5/8'' so for the 1.5'' liner I just use a plastic bushing the has the proper ID and OD and stuffs in the end of the spindle. I made a screw on cap that secures the liner into the spindle, but my spindle was already threaded, I don't know if the Slant Pro has that thread. The length of the liners was dictated by what I could fit into my 13x40 manual lathe I would have made them a bit longer had I been able to do so.
For much longer material, a support stand is required. Take a look at the Haas bar feeder.
https://www.haascnc.com/machines/lathes/bar-feeder.html
If you take out the rest of the hardware you are left with a piece of plastic lined angle iron on a couple of screw jacks, pretty simple to build a similar arrangement. If I had the room I would build one for my lathe so I could run bar lengths.