You could try combining a couple of the suggestions above to get very close to a floor press. First, using just a bar see how far off your chest the bar would be if you were doing floor presses. Then get enough boards to equal this distance. Then do feet up bench press with this number of boards.
An alternative, if your gym doesn't mind you bringing in your own bits of equipment, is to use a pair of adjustable car jacks as a mini-rack to hold the bar. You need to be careful racking the bar to make sure you don't knock over one of the jacks, though.