I was in Decathlon yesterday and found a compression bag which has a vent (similar to the ones found on self inflating mats). You roll the top of the bag to seal it, then squeeze the sack to expel the air out of the bag to reduce the pack size. This bag is heavy duty, waterproof and much thicker than standard vacuum bags which tend to split or leak air very easily.
A 45ltr bag which is more than big enough for a sleeping bag costs £19.99
I need your help to find a similar but cheaper alternative, ideally for the same size (45ltr) or do you think this is a good price?
Please do not recommend vacuum bags (tend to split or leak very easily).
I use plain old dry bags in the same way. Same idea as the compression bag but no valve. The technique is to make a single fold at the closure of the bag, lean/sit/kneel on it to squash all the air out, then fold the closure over and clip it to seal.
If it's just for a single sleeping bag, I'd consider something smaller than 45 litres. As a rough guide, my down bag goes in an 9 litre dry bag (orange), about £5. For a synthetic or square bag I'd use the next size up in Exped bags, which is 14 litres (pale blue), about £6.
We use these Alpkit Dry Bags for all sorts of things. £14 for 65 litre - they come in other sizes.
Kneel on them to squish them down and then seal the tops.
We use them for pillows, towels, bedding, coats and all sorts of things so that nothing is loose in the trailer and if we pack/unpack in the rain it all stays dry.