you may try a Rex around them - they have the shortest fur (hair is usually the culprit for allergies). are you certain they're allergic to rabbits or maybe just hay? if hay is the allergy you can purchase cubes of hay - it's just compressed hay so it's not dusty
my buns have their own room, but a circle pen is another great idea for buns to make sure they have plenty of space if you don't want to give them the run of the room.
how long would you be leaving the bun alone for? if it's only a day or two, he'd be ok, just make sure you give him extra water, hay, and food, and if you can, have someone to come by and look in on him.