Carrots in pots undercover

I'm trying carrots in pots for the first time and have sown some Amsterdam  Forcing in a couple of MFBs to eventually go undercover to extend the season.  What soil have those who do this regularly found to be best.  Ordinary sieved garden soil, bought compost, or a mix.  I'm assuming undiluted home made compost would be a bit rich.

I'm also doing this for the first time but will revert to a RB next year

Broadway, what's an RB?