Backpack:
A backpack with ski carry straps will work. It needs 4 straps. If you go this route, you'll just need the Velcro straps below.

DIY Backpack:
Or you can buy a sprayer backpack and adjust it to fit... The nice thing about this is the "back pack" is flat without the bike and can easily be put on under a jacket and you won't notice it other than it seems to help me keep better posture.

Supplies

I used this with very small adjustments:
Smith Performance Sprayers 182943 Padded Nylon Backpack Straps https://www.amazon.com/dp/B01MY1YLJZ/ref=cm_sw_r_cp_apap_MxUfbFHn9ceKy

Velcro straps:
Velour Cinch Straps 1.5" x 15" -... https://www.amazon.com/dp/B07FY72GWX?ref=ppx_pop_mob_ap_share

Extra clips for DIY backpack:
SGH Pro Quick Side Release... https://www.amazon.com/dp/B07PQT995L?ref=ppx_pop_mob_ap_share

Step 1: DIY Backpack Adjustments

Using the Sprayer backpack, all you need to is put the extra clips on the bottom straps.

They won't stay on their own; so you'll just need to put the clip on the strap then sew a fold into the straps to pevent the clip from coming off.

Step 2: Strap the Bike (Protection)

Note... If you want your bike to stay in pristine condition, you are going to want to more than this... But just putting one or two straps along this point on the bike will prevent the bottom from scratching the frame.

Step 3: Continue Strapping

Fit the main frame to the bottom as shown in the pictures...

I use two straps at the center of the frame then one at the bottom. The straps need to go both around the frame and the nearest bottom section.

Step 4: Bike to Backpack

Mount the bike frame to the backpack at the shown mounting points.

Step 5: That Pesky Front Ski

Strap that to bottom of one other skis...

Step 6: Handle the Handlebar

Depending on your setup, it might be different.

I just wrap my leash around the handle bar and the closest ski as shown...

Step 7: Get You Lift On

Lift that beast and put it on!

Enjoy the cheap lots!

I find it easier to take it off and hole the bike by the front fork and handle bar when loading on a bus.