You get a box, or a tray, or a ziplock container, or etc.., metal on the bottom, then put magnets on the bottom of bases (n52 magnetics work best for large models).
You could go costly and get Magna Racks from Battlefoam.
I bought water/shield tight Ziplock containers (about 18"W x 24"L x 9"H) edit: around that size, i'm guessing base off memory, and put sheet meta on the bottom of it (I had craft foam sheet under the metal 1st) over all cost me 15$ per set, i have 8 of them like that for my armies. All my 4k ish Harlequins points fits in 1.
EDIT: this is what i'm talking about, you can get them much cheaper, i just found the right size, i paid $11USD each for mine
https://www.amazon.com/Ziploc-WeatherShield-Quart-Storage-Clear/dp/B01M9CHXGX/ref=sr_1_1?keywords=ziplock+shield+6.6+gal&qid=1573159816&sr=8-1