EZ
When I built my trailer the first attempt at putting on the dust caps, one cap was so tight I couldn't get it on. the other side was so loose I could put it on with just my hand.
So I switched sides. Now the tight side would fit and the loose side had a better fit but was still not tight enough. So I peened around the inside edge of the cap with a center punch to put these little dents on it. That made it fit a lot better.
On my first trip I lost one of the caps. As it turned out, the cap that had the best fit, (the one without the peened dents), was the one that came off.
To temporarily cover the hub I used duct tape and one of these Styrofoam coffee cups that you get at one of these self service gas stations, the small coffee cup worked pretty good. Once I got to my destination I found that one of these small plastic yogurt cups worked better.
Make sure you eat the yogurt first.
I now keep a couple of these in the camper just in case I loose another cap.
Check out page 9 of my album to see a home made tool you can use to put on dust caps without denting them. The one in the picture is a short piece of 2" plumbing pipe with a pipe cap on one end. I would take your dust cap to the hardware store to check the pipe diameter if you want to make one these. Just hit the end of the pipe with a hammer and your dust cap is on without any dents.