Well, this isn't exactly going to be a "neat trick".
I have the same problem with squirrels, and foxes are even worse. What I did on squirrels was to spray my clothes with flea repellent to keep them off me, and skin someplace away from the house. Skinning gets rid of all but the ones that jump off, and if you're sprayed good they won't stay on you.
For foxes (I didn't eat them), I carried large plastic garbage bags and a flea-killing spray in the truck. Took the bag to the trap and put the fox in it, when I got to the truck I opened it a little bit and shot some flea spray inside. After about an hour I'd get the fox out, and it sounded like sand hitting the floor of the skinning shed when I shook the fleas out. I got very few bites that way.
The old Nessmuk formula (pennyroyal, pine tar, and castor oil) for "bug dope" seems to work in the woods to keep them away. It's messy, but it works for me.
According to Ian Fleming, the Japanese poet Bassho wrote, "A certain number of fleas is good for a dog ... otherwise, he forgets he is a dog". The problem I face is that my wife does not agree with Bassho, and gets most peculiarly upset if I bring any of the little attitude-adjusters to our bed.