There's adjustable size / one size fits most U-Lock holsters out of fabric. Something like this, albeit you might wanna add some padding on the downtube in this example, an old inner tube or something. Also available for rear racks, easy enough to find, probably, on whatever local online shopping is available to you with the term "u-lock holster"

Alternatively if you're willing to tinker there's this idea, originally by /u/Bipro1ar on reddit who I do give credit because it's sick as fuck.

That one might involve some metalwork though, albeit I do think retrofitting a commercially available front rack with thicker PVC-pipes in this style of idea should be feasible with some gumption and a bunch of zipties.