Buy cheap new suitcase. Add contents wrap in cardboard sealed with brown tape.... you now have a "box"....
leave a slot with the handle projecting, you now have a cardboard-sheathed suitcase.( the cardboard, justifiably, protects your case from scuffs/damage.
Let the pedantic jobsworths spend their (bored) time and their boss' money arguing the minutiae of what makes a suitcase "not" a box-with-a-handle
Common-sense has flown right out of the window..."security" is a sham theatre by and for, a self-serving burocracy. it's rife with inconsistencies, petty tyrants abusing authority whilst being unanswerable.......GRRRR.
Perhaps if, like some fuel-station/ fast-food/ supermarket cashiers who are personally liable for shortfalls on their shift. -there was an appeals procedure, instant and independent,for aggrieved Pax and the "security Officer" was PERSONALLY responsible for "mistakes".........
I'm sure there would be a much more tempered approach to "on the spot" arbitrary confiscations/bans etc.
Last flew about 5 years ago and I, too , had a non-standard bag.....quick, simple and no hassle..."just take it to the far end where the sign says......"
no probs at Liverpool.....but the petty hassle and timewasting for the theatricals means in future i'll probably drive or most likely not bother again.