One of the easiest DIY toys for a rabbit is take an old toilet paper or paper towel roll and pack it full of hay. You can then poke some holes in the side or not, depending on the critters interest. Since you are giving to a shelter, you can poke a hole in the middle and run some twine so they can hang it from the cage. I like to hide some dehydrated or freeze dried fruit chips in the hay to give a little treat along with it (I get unsulfured dried fruit from Trader Joe's, just read the label and make sure no added sugar). Usually not more than one piece per inch of tube. You can either make slits in the end to fold over the ends or leave them open. Folding can be more fun for chewing but my rabbit prefers the hay sticking out the end.
You can take the same empty rolls and cut them into rings. Then stick the rings inside each other (2-3) to make a ball. You can stick hay in the ball or leave it empty. Fun little toss toy and chew toy.
Or.... Same empty tubes poke holes along the sides and poke (safe) twigs through the tube. Great if you have an apple tree because they love that one, just be sure no pesticides and guinea pig safe.
A variation on the above is get empty cardboard boxes (which many grocery stores will let you take) and cut holes in the sides for entry. They might be too big for most shelters small animal areas though. But you can get super creative with cardboard. You can also stuff them full of hay. You might be able to ask discount shoe stores for their shoe boxes that people dont take for a smaller version