I have to give credit to dlginstructables for the idea shown here: https://www.instructables.com/id/DIY-Bicycle-Pannie...; but I wanted something a little different.

What I wanted:

A single sided cheep (I MEAN CHEEP) pannier for shopping that was always attached to my bike, but didn't get in the way with everyday commute when not in use.

Materials and cost:

Tent peg (2x) (rounded hook) - £0.40

Hessian (fabric) bag - £2.00

Zip tie (2x) - £0.04

Hardboard (3mm thick) - £1.50 (Big piece at B&Q £3, don't even need half)

Elastic cord (3mm dia) - £0.50

Split key ring - £0.70

Tape - £0.10

TOTAL: £5.24 - Honestly you should be able to source most of this stuff around your house. I only had to buy an off cut of hardboard.

TOOLS:

Less common tools: Staple Gun, Vice (hard to do without one), Drill + bits

Common tools: Wood saw, Hack saw, Hammer, Ruler, Pencil, Sissors

CHALLANGE FOR READER AND NOTES ON LEAVING COMMENTS: Come up with ideas to make it cheaper (but same or better strength), stronger (but same or cheaper cost), Simpler (with no loss of functionality). When I say cheaper, I do not mean the same materials just sourced cheaper than I stated! Remember it is easy to criticize; but I realize that it is important in growth of an idea. FEEL FREE TO TARE FLAWS IN THE DESIGN, BUT ALWAYS LEAVE A SUGGESTION ON HOW YOU WOULD FIX IT FOR OTHERS TO FOLLOW. There are of coarse trade offs, and I don't think people need these pointing out...it is cheep, it will not last the same life as a bought pannier, it is cumbersome, it is an eye saw. Now we have established this, it will limit the negativity in the comments. Have fun experimenting... =]

Step 1: Step 1: Backing the Bag...

I will not give dimensions as almost all bike racks and fabric bags vary.

Cut a square of hardboard so that it fits snug inside the bag against one of the bigger faces (this will be the side that rests on the rack).

Cut the square in half (horizontally), then trim another inch off the lower half horizontally. You can see the two halves in the picture (bigger top, smaller bottom - allows folding of the bag).

Staple the Hardboard to the bag...MAKE SURE TO LEAVE A GAP BETWEEN THEM...the top edge of the top board should be against the rim of the bag...the lower edge of the lower board should be against the bottom of the bag.

Hammer staples flat (hammer at an angle to stop them popping back out). You can use thicker hardboard if you wish, but I prefer a lighter smaller bag, and just tape over the staples. CAREFUL THEY ARE SHARP.

Step 2: Step 2: Hooks

Bend the tent pegs back on them selves using the vice and hammer (can be a bit fiddly).

Saw off excess material (see picture).

Drill hole in hardboard backing at the appropriate place for the bottom of the hook to rest (the original hook of the peg will go over the rack, the bent part will go through this hole. Drill hole either side of peg for cable ties (see pic).

Note: to get pegs through the hole, poke them in a bit, then hammer them in the rest of the way, the hardboard will give a little to allow them through.

Step 3: Step 3: Bottom Loop

Tie each end of a length of elastic to each hook on the inside.

Drill a bigger hole near the bottom and poke a bite of the elastic through the hole...attach the split key ring to the loop to stop it being puller back through.

Finish off the inside of the bag with tape to remove all sharp hazards...your done.

To attach, loop the bottom ring over the bottom of your rack, and tension the elastic until the hooks engage on the top of your rack. When not in use, fold in half and strap to the top of your rack with bungee cord.

NOTE: YOU MAY WANT TO CLOSE THE PEGS OF SLIGHTLY WITH A HAMMER TO MAKE THEM FIT THE RACK BETTER.

Sorry for lack of pictures through the steps, I decided to write this after I built it and did not have enough pegs to build another. Hope you have fun.