3 foot instrument cables?

Cut a longer spare cable. A soldering iron might be cheaper than buying a one foot cable, too. The connectors are VERY easy to solder.

A whole BUNCH of people make those...
https://www.amazon.com/s/ref=a9_asi_1…nstrument+cable

Of course, you can make it the exact length you need if you make it yourself.

I order the lengths I need (for both XLR and 1/4" phone TS or TRS) from Evidence Audio

http://www.evidenceaudio.com/product/the-melody-mlss

I ordered some 3 foot Road Hog cables from Sweetwater a month or so ago. There's actually a pretty wide selection out there on the interweb.

But...

why don´t you simply use the alternative input in the back of your Kemper?

That’s a really good point. I use the alternate input on the back of mine.

Of course, if the rack is a really tall one, a long cable may be needed anyway.

For good value I've always bought Hosa.

Everything you'll ever need length and connector-type wise.

That Input is not all that noisy. For me, the Alternative Input is more than quiet enough for playing live Rock shows.

When I record, though, I use the front input.