For bikepacking, starting out, I used some school book type daypacks as imporvised panniers.

I modified them by:

1) cutting off straps.
2) putting in a reinforcing thin panel of plywood or polycarbonate type plastic as a "frame" .. the type of construction in common bicycling "grocery bags". Go take a look at them in bike stores or online.
3) I copied the attachment dimensions off of a bicycle "grocery bag" and this involved some hardware style hooks to hang from the top of the bike rack as well as a section of elastic and a snap ring to attach to the bottom of the bike rack.
4) The bags were about $10 each and the parts per bag were on the order of $5.00 each. The total capacity of the two bags was nearly 3,000 cu. in. and there was still the option of strapping the tent and sleeping bag to the top of the bike rack if you had too much stuff.

I didn't care for the unbalance of the loaded backpack on the bike and I really didn't like the sweaty back that accompanies wearing a backpack on a bike.

Eventually, I bought a bicycle trailer for conveniently hauling all my bikecamping gear. With more skill in lightweight backpacking, I could probably go back to the improvised pannier approach.