You can find bin (garbage bags) in the local tienda. Share with others in a similar predicament. Use one inside your rucksack to contain all your stuff, as a rucksack liner.
Take a second bin bag and make slits for the straps. Place this bag over the exterior of your rucksack with the opening to the bottom. There should be four strap holes/slits. Two at the upper shoulder, and two where the bag reconnects to the lower part of the rucksack.
Use some of the duck tape you SHOULD be carrying to reinforce the four slits you made to pass straps through. I usually cover the slit, place a second, overlapping layer over that, then "reopen" the slit using my pocket knife or the tiny scissor on my TSA-friendly multi-tool.
FYI, heavy gauge lawn and leaf bags, or contractor bags are more durable. If there is a ferreteria nearby (hardware store) try there. But, if you had to buy a box of say ten bags, you have spares to share with others. REMEMBER TO DISPOSE of used bags responsibly.
BTW, the largest bags also make good field-expedient rain coats to cover at least your torso and help keep the heat in, to prevent hypothermia in colder temperatures.
I hope this helps.