Frontside pop shove-it late flip is a valid reply, it's essentially a delayed varial heelflip using the toes; while not that much harder (personally I learned it first), backside I'd say takes slightly more precision in the timing and technique. Late (back foot) heelflip also is really easy, you start a kickflip but then instantly kick out with your back leg to counter it. Kickflip counterpart is more subtle having to use the toes all the while keeping the board under you whereas anything late heelflip you can sort of kick and hope away. Late varial flip may be easier than straight late flip too because you barely need an actual ollie for those, you can just pop the tail then instantly kind of sweep the concave around the bolts.
Easiest I'd say is half impossible (or half pressure flip) late half flip (using the front foot), very 1992 trick but it just works, basically a pop shove-it late flip where the board gets inverted and taking half the effort. You basically pop then catch the graphic side with your front foot and pull it back towards you, board just wants to flip over then.
Late varial underflips are also easy - pop a normal ollie but reach down with the back foot toes and pull up somewhere around the heel-side back wheel to send the board around like a varial heelflip, it's basically a modified late shove. Only hurts your toes if you hit it wrong. Fakie bigspin underflips I also remember finding easier than pop shove-it underflips for a bit as a kid.
'True' casper flips using both feet à la Templeton are a whole different ball game than the front foot only version.