My solution was a Torin TF0202 double ram bottle jack. Very compact and extends from 5 1/4" high to 14" high and has 4K lb capacity. Doesn't seem to be available anymore but there are other similar brands. Fits under a U-bolt with a block of 2x4 on top of the jack (even with a flat). Easy to store in our under-bed storage. Better than jacking under an I-beam or using the Trailer Aid I think because the trailer won't get tilted to one side, but you must be very very careful if ever jacking up an axle and must have a block of wood under the U-bolts (never under the axle tube). Besides using a jack for a flat, they're also good for inspecting brakes and bearings.