why? you just need something between your but and your saddle that does not make you skip or bail your workout. For some it’s thick padding, for others… whatever.
I just use regular bibs, that I also use outside, as long as they are comfortable 
keep an eye on wear, as you probably (should) wash your kit more with frequent short workouts instead of a few long outdoor rides, so your kit can wear out quicker when using indoor.
I prefer a bib short because it the chamois stays better in the right place, I also use a thin base layer shirt as it helps with better sweat regulation.
I really don’t see the need to search for an alternative without additional information