My underlayer isn't completely straight, but it'd barely classify as 2a on most days, and it is extremely resilient to heat styling(particularly curling irons). It's summer and I've been trying to get away from using so many products(it's just kind of a pain), usually just leave in or curl milk after I shower, occasionally some coconut oil. I imagine if you're underlayer really is just naturally straight, then you'll have about as much luck as someone with straight hair using products to get curl, though if you haven't you may want to give CG a go.
A few things you could try would be braiding/plaiting the underlayer while it's still damp. I would only recommend 1 -2 braids, so that the waves don't come out tight and unnatural looking, and despite what some people say, they need to be even sections and neatly braided(not really perfect, but you're using the braids to shape waves and you wouldn't want crooked uneven waves), and the less you leave as the "tail" of the braid the less straightness you'll have at the end. A slightly easier way is to find flexirods that are about the same size as your natural wave/curl pattern, and use them on your underlayer while damp, and take them out when it's dry. If those don't work out you can always get the bottom layer trimmed shorter as mentioned above.
Hope you figure something out.