Gap under (double) garage door

[someonelse]

The drive has a slight slope to one side so though the door closes pretty tight to the ground on the left, on the right there's a gap.

I've been offered a rubber weathershield to attach to the concrete drive (20mm - £77.52 + Vat). Is that a reasonable cost to supply /fit, or how easy would it be to DIY fit, and what exactly would I buy (plus cost)?

 

[sircerebus666]

https://www.toolstation.com/stormguard-garage-door-threshold/p27422

Something like this which you simply glue down (depends on the height of the gap)